JP2011035696A - Slave terminal, and method for correcting clock of the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a slave terminal that facilitates synchronization with master time of a master terminal. <P>SOLUTION: The slave terminal includes: a subcounter 302 which adds tick rates together for each time interval to calculate subcount; a main counter 303 which increments main count when the subcount reaches a threshold; an error calculation unit 305 which calculates a time error between non-corrected slave time to be calculated from the main count and the master time of the master terminal when non-corrected tick rate is used; a correction value calculation unit 306 which calculates a value of a correction variable at which correction time difference between corrected slave time to be calculated from the main count and the non-corrected slave time becomes equal to the time error when the correction variable is added to the tick rate; and a clock correction unit 307 which adds the calculated value of the correction variable to the tick rate. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a network technology, and relates to a slave terminal and a clock correction method for the slave terminal.

  As measurement and control systems become more complex and support a large number of nodes, there is a growing need for distributed network technology. Here, it is important to maintain a local clock in each node and to maintain an accurate time in the entire distributed network (see, for example, Patent Document 1). However, when the power supply of each node is individually turned on / off, there may be a deviation from an accurate time in each local clock. Therefore, in order to maintain accurate clock timing throughout the distributed network, it is necessary to synchronize individual local clocks with respect to the accurate clock.

JP 2006-319972 A

  However, when the time difference between the local clock and the accurate clock is large, it may be difficult to achieve synchronization. Therefore, an object of the present invention is to provide a slave terminal and a slave terminal clock correction method that can be easily synchronized with the master time of the master terminal.

  According to an aspect of the present invention, a slave terminal that can be connected to a master terminal via a transmission path, calculates a subcount by adding a tick rate at each time interval, and the subcount reaches a threshold value. In this case, a main counter that increments the main count, and an error calculation unit that calculates a time error between the uncorrected slave time calculated from the main count and the master time of the master terminal when an uncorrected tick rate is used. A correction value calculation unit that calculates a correction variable value at which a correction time difference between a correction slave time calculated from a main count and an uncorrected slave time is equal to a time error when a correction variable is added to the tick rate; A slave terminal is provided that includes a clock correction unit that adds the value of the calculated correction variable to the tick rate.

  According to another aspect of the present invention, a subcount is calculated by adding a tick rate at each time interval, a main count is incremented when the subcount reaches a threshold, and a transmission path is used. To obtain the master time from the master terminal, to calculate the time error between the uncorrected slave time calculated from the main count and the master time when an uncorrected tick rate is used, and to correct the tick rate When a variable is added, calculate the value of the correction variable so that the correction time difference between the corrected slave time calculated from the main count and the uncorrected slave time is equal to the time error, and the calculated correction variable value is ticked. A method for correcting the clock of the slave terminal, including adding to the rate.

  ADVANTAGE OF THE INVENTION According to this invention, the clock correction method of the slave terminal and slave terminal which can be easily synchronized with the master time of a master terminal can be provided.

1 is a schematic diagram of a computer network according to a first embodiment of the present invention. It is a schematic diagram of the slave terminal which concerns on the 1st Embodiment of this invention. It is a graph which shows typically the relation of subcount and main count concerning the 1st embodiment of the present invention, and time. It is a schematic diagram which shows the relationship between the message transmitted to the slave terminal from the master terminal which concerns on the 1st Embodiment of this invention, and time. It is a flowchart which shows the correction method of the clock of the slave terminal which concerns on the 1st Embodiment of this invention. It is a graph which shows transition of the clock time data of the master terminal and slave terminal which concern on the comparative example of the 1st Embodiment of this invention. It is a graph which shows transition of the clock time data of the master terminal and slave terminal which concern on the 1st Embodiment of this invention. It is a flowchart which shows the correction method of the clock of the slave terminal which concerns on the modification of the 1st Embodiment of this invention. It is a schematic diagram of the slave terminal which concerns on the 3rd Embodiment of this invention. It is a schematic diagram which shows the relationship between the message transmitted between the master terminal and slave terminal which concern on the 3rd Embodiment of this invention, and time. It is a graph which shows transition of the clock time data of the master terminal and slave terminal which concern on other embodiment of this invention.

  Embodiments of the present invention will be described below. In the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals. However, the drawings are schematic. Therefore, specific dimensions and the like should be determined in light of the following description. Moreover, it is a matter of course that portions having different dimensional relationships and ratios are included between the drawings.

(First embodiment)
As shown in FIG. 1, the slave terminal 10 according to the first embodiment can be electrically connected to the master terminal 1 via the transmission path of the network 5. The master terminal 1 is connected to an accurate time source such as a global positioning system (GPS) or an atomic clock, and has a master clock capable of providing an accurate master time. The slave terminal 10 includes a central processing unit (CPU) 300 including a port 301 shown in FIG. 2 for transmitting / receiving data to / from the master terminal 1 and a slave clock 201. The slave clock 201 is based on a subcounter 302 that calculates a subcount by adding a tick rate at each time interval, a main counter 303 that increments the main count when the subcount reaches a threshold, and a main count. And a time calculation unit 304 for calculating the slave time.

  Further, the CPU 300 calculates the time error between the uncorrected slave time calculated from the main count when the uncorrected tick rate is used and the master time of the master terminal 1 shown in FIG. 305 and the value of the correction variable that makes the correction time difference between the corrected slave time calculated from the main count and the uncorrected slave time equal to the time error calculated by the error calculation unit 305 when the correction variable is added to the tick rate And a clock correction unit 307 that adds the calculated value of the correction variable to the tick rate of the sub-counter 302.

  In the sub-counter 302 of the slave clock 201, as shown in FIG. 3, a predetermined tick rate A is added every fixed time interval ω to calculate a sub-count Cs. The fixed time interval ω is, for example, 20 nanoseconds. The value of the tick rate A is determined based on the resolution requirement for the slave clock 201 in consideration of the time interval ω and the period Ω of the main count Cm described later. When the subcount Cs exceeds the predetermined threshold value R, the main counter Cm shown in FIG. 2 is incremented by 1 as shown in FIG. Each time the subcount Cs exceeds the threshold value R, the value of the threshold value R is subtracted from the value of the subcount Cs. Thereafter, every time the subcount Cs exceeds the threshold value R, the main count Cm is incremented by one.

A correction variable N can be added to the tick rate A of the sub-counter 302. When the correction variable N having a positive value is added to the tick rate A, the time until the subcount Cs exceeds the threshold value R is shorter than when the correction variable N is not added to the tick rate A. Therefore, the cycle Ω at which the main count Cm is incremented is also shortened. In addition, when the negative correction variable N is added to the tick rate A, the time until the subcount Cs exceeds the threshold R is longer than when the correction variable N is not added to the tick rate A. Therefore, the period Ω at which the main count Cm is incremented also becomes longer. The period Ω in which the main count Cm is incremented is given as a function of the correction variable N as shown in the following equation (1). In addition, when the period Ω in which the main count Cm calculated from the following equation (1) is incremented includes a fractional part, the fraction may be rounded down.
Ω (N) = Rω / (A + N) (1)

The time calculation unit 304 shown in FIG. 2 increments the main count Cm of the main counter 303 when the correction variable N is not added to the tick rate A of the subcount of the subcounter 302 as shown in the following equation (2). The slave time Ts is calculated by multiplying the cycle Ω (0) by the main count Cm.
Ts = Cm × Ω (0) (2)

  The master terminal 1 illustrated in FIG. 1 periodically transmits the “synchronization message” and the “follow message” illustrated in FIG. 4 to the slave terminal 10. When transmitting the “synchronization message”, the instruction unit in the master terminal 1 instructs the port in the master terminal 1 to transmit the “synchronization message”. Further, the instruction unit inside the master terminal 1 acquires the master time Tm_t1 when the “synchronization message” is actually transmitted from the port from the master clock. Further, the instruction unit in the master terminal 1 instructs the port to transmit a “follow message” to which the master time Tm_t1 is given.

  The “synchronization message” and “follow message” are received at the port 301 of the slave terminal 10 shown in FIG. In a state where the correction variable N is not added to the tick rate A of the sub-counter 302 of the slave clock 201, the error calculation unit 305 uses the slave clock 201 as the uncorrected slave time Ts_r1 when the port 301 receives the “synchronization message”. Get from. Further, the error calculation unit 305 acquires the master time Tm_t1 from the “follow message”.

The time error E between the master time Tm_t1 held by the master terminal 1 shown in FIG. 1 and the uncorrected slave time Ts_r1 held by the slave terminal 10 indicates the transmission path delay from the master terminal 1 to the slave terminal 10 in the network 5 as Dms. Is given by the following equation (3).
E = Ts_r1-(Tm_t1 + Dms) (3)
In the first embodiment, the error calculation unit 305 shown in FIG. 2 sets the time error E on the assumption that there is no transmission line delay Dms from the master terminal 1 to the slave terminal 10 as shown in the following equation (4). calculate.
E = Ts_r1-Tm_t1 (4)

The CPU 300 further includes a determination unit 308. The determination unit 308 determines whether or not the time error E is within an allowable range. The allowable range is arbitrarily set in advance by an operator or the like. Here, during a predetermined adjustment time τ, the difference between the main count when the correction variable N is not added to the tick rate A of the sub-counter 302 and the main count when the correction variable N is added to the tick rate A. ΔCm is given by the following equation (5).
ΔCm = τ / Ω (0)-τ / Ω (N) (5)
From the above equation (1), the equation (5) can be modified as the following equation (6).
ΔCm = τ / Ω (0)-τ / Ω (N)
= τ / (Rω / A)-τ / [Rω / (A + N)]
= [τ (A + N)-τA] / (Rω)
= (Nτ) / (Rω) (6)

Further, during a predetermined adjustment time τ, the uncorrected slave time calculated when the correction variable N is not added to the tick rate A, and the corrected slave calculated when the correction variable N is added to the tick rate A The corrected time difference ΔTs from the time is given by the following equation (7).
ΔTs = ΔCm × Ω (0) (7)
From the above formulas (1) and (6), the formula (7) can be transformed into the following formula (8).
ΔTs = ΔCm × Ω (0)
= (Nτ) / (Rω) × (Rω / A)
= Nτ / A (8)

When the determination unit 308 determines that the time error E is outside the allowable range, the correction value calculation unit 306 occurs between the time error E and a predetermined adjustment time τ as shown in the following equation (9). The value of the correction variable N that makes the correction time difference ΔTs equal is calculated.
E = ΔTs (9)
From the above formulas (8) and (9), the value of the correction variable N at which the time error E is equal to the correction time difference ΔTs generated between the predetermined adjustment time τ is expressed by the following formula (10). It can be calculated. Note that the value of the predetermined adjustment time τ is set to an arbitrary value in advance by an operator or the like.
N = A × E / τ (10)
By adding the value of the correction variable N calculated from the equation (10) to the tick rate A of the sub-counter 302 for a predetermined adjustment time τ, the time error E can be zero. Specifically, the time τm that the master clock advances during a predetermined adjustment time τ is equal to the time τs that the slave clock 201 whose main counter cycle is Ω (0) advances during the predetermined adjustment time τ. When the value of the correction variable N calculated from the equations (10) is added to the tick rate A of the sub-counter 302, the time error E becomes zero.

In addition, the time τm that the master clock advances during the predetermined adjustment time τ may not be exactly equal to the time τs that the slave clock 201 advances during the predetermined adjustment time τ. However, as shown in the following equation (11), compared with the time error E, the difference between the time τm and the time τs is usually very small.
E >> (τm-τs) + α (11)
Therefore, the time error E can be sufficiently reduced after the predetermined adjustment time τ. In the equation (11), α is an error factor such as an estimation error of the transmission line delay.

  Further, when the transmission line delay Dms from the master terminal 1 to the slave terminal 10 in the network 5 shown in FIG. 1 is a value that cannot be ignored, the time value can be obtained even if the value of the calculated correction variable N is added to the tick rate A. The error E may not be zero. However, by adding the calculated value of the correction variable N to the tick rate A, the time error E can be reduced as compared with the case where the value of the correction variable N is not added to the tick rate A.

  The clock correction unit 307 illustrated in FIG. 2 corrects the slave clock 201 by adding the value of the correction variable N calculated by the correction value calculation unit 306 to the tick rate A of the sub-counter 302 for a predetermined adjustment time τ. After a predetermined adjustment time τ has elapsed, the clock correction unit 307 stops adding the value of the correction variable N to the tick rate A of the subcounter 302. That is, after a predetermined adjustment time τ has elapsed, the value of the correction variable N is set to zero.

When the determination unit 308 determines that the time error E is within the allowable range, the correction value calculation unit 306 may calculate the value of the correction variable N using the following equation (12). In this case, the clock correction unit 307 performs proportional-integral (PI) control by adding the value of the correction variable N calculated using equation (12) to the tick rate A of the sub-counter 302.
N = E / P + Σ (E / I) (12)
Here, P represents a proportional band, and I represents an integration time.

  A parameter storage device 350 and a corrected time difference function storage device 351 are connected to the CPU 300 shown in FIG. The parameter storage device 350 stores the value of the adjustment time τ set by an operator or the like. Further, the parameter storage device 350 stores data of an allowable range of time error between the corrected slave time and the master time set by an operator or the like. Further, the corrected time difference function storage device 351 stores the above equation (8) as a corrected time difference function. An input device 312, an output device 313, a program storage device 330, and a temporary storage device 331 are further connected to the CPU 300. As the input device 312, a keyboard, a mouse, or the like can be used. As the output device 313, a monitor screen using a liquid crystal display (LCD), a light emitting diode (LED), or the like can be used. The program storage device 330 stores a program executed by the CPU 300. The temporary storage device 331 temporarily stores data in the calculation process of the CPU 300.

  Next, a method of correcting the clock of the slave terminal according to the first embodiment will be described using the flowchart shown in FIG. Note that the calculation results of the CPU 300 shown in FIG. 2 can be sequentially stored in the temporary storage device 331.

  (A) In step S100, the power of the slave terminal 10 shown in FIG. 1 is turned on. Thereby, the synchronization process of the slave terminal 10 with respect to the master terminal 1 is started. The sub-counter 302 shown in FIG. 2 calculates a sub-count Cs by adding a predetermined tick rate A every fixed time interval ω as shown in FIG. Each time the subcount Cs exceeds a predetermined threshold R, the main counter 303 shown in FIG. 2 increments the main count Cm by 1 as shown in FIG. Also, the time calculation unit 304 shown in FIG. 2 calculates the uncorrected slave time using the above equation (2).

  (B) In step S101, the slave terminal 10 stands by until it receives a “synchronization message” from the master terminal 1 shown in FIG. When the port 301 illustrated in FIG. 2 receives the “synchronization message”, the port 301 transmits the “synchronization message” to the error calculation unit 305. In step S102, the error calculation unit 305 acquires the uncorrected slave time Ts_r1 when the port 301 receives the “synchronization message” from the slave clock 201.

  (C) In step S103, the slave terminal 10 stands by until a “follow message” is received from the master terminal 1 shown in FIG. When the port 301 illustrated in FIG. 2 receives the “follow message”, the port 301 transmits the “follow message” to the error calculation unit 305. In step S104, the error calculation unit 305 acquires the master time Tm_t1 from the “follow message”. Next, in step S105, the error calculation unit 305 determines that there is no transmission path delay Dms from the master terminal 1 to the slave terminal 10 in the network 5 shown in FIG. Then, a time error E from the uncorrected slave time Ts_r1 is calculated. Thereafter, the error calculation unit 305 illustrated in FIG. 2 transmits the calculated time error E to the determination unit 308.

  (D) In step S106, the determination unit 308 reads the allowable range data from the parameter storage device 350, and determines whether the time error E is within the allowable range. If the time error E is outside the allowable range, the process proceeds to step S107. In step S107, the correction value calculation unit 306 generates a correction time difference function that gives a correction time difference ΔTs between the uncorrected slave time and the correction slave time, which is given by the above equation (8), from the correction time difference function storage device 351. Read it out. Further, the correction value calculation unit 306 reads out the value of the adjustment time τ previously stored by the operator or the like from the parameter storage device 350. Next, the correction value calculation unit 306 substitutes the value of the time error E calculated by the error calculation unit 305 for the variable of the correction time difference ΔTs of the correction time difference function of equation (8). Further, the correction value calculation unit 306 substitutes the value of the adjustment time τ read from the parameter storage device 350 for the variable of the adjustment time τ of the correction time difference function, and is specific to the subcounter 303 for the variable of the tick rate A. The value of the correction variable N is calculated by substituting the value of the tick rate A. The correction value calculation unit 306 transmits the calculated value of the correction variable N to the clock correction unit 307.

  (E) In step S108 and step S109, the clock correction unit 307 adds the value of the correction variable N calculated by the correction value calculation unit 306 to the tick rate A of the subcounter 302 during the adjustment time τ. The subcounter 302 calculates the subcount Cs by adding the tick rate A to which the value of the correction variable N is added every time interval ω. When the tick rate A is corrected, the main counter 303 increments the main count Cm with the corrected period Ω (N). Further, the time calculation unit 304 calculates the corrected slave time based on the main count Cm that is incremented with the corrected period Ω (N). After the adjustment time τ has elapsed, the clock correction unit 307 resets the value of the correction variable N to be added to the tick rate A of the sub-counter 302 to 0 in step S110. Thereafter, in step S130, it is determined whether to continue the synchronization process of the slave terminal 10 with respect to the master terminal 1. If the synchronization process is continued, the process returns to step S101.

  (F) When the determination result in step S106 is within the allowable range, the process proceeds to step S120, where the correction value calculation unit 306 calculates the value of the correction variable N using the above equation (12), and the clock correction unit 307 The sub-counter 302 is corrected by a PI control method having excellent steady characteristics. Thereafter, in step S130, it is determined whether to continue the synchronization process of the slave terminal 10 with respect to the master terminal 1. If the synchronization process is continued, the process returns to step S101. When it is not necessary to continue the synchronization process, such as when the slave terminal 10 is turned off, the synchronization process is terminated.

  Conventionally, after the slave terminal is turned on as shown in FIG. 6, the first synchronization message and the follow message are received from the master terminal, and the time error between the master time and the slave time is eliminated only by the PI control method. There was a willingness. However, since the time error immediately after the power is turned on is large, when trying to correct the time error by the PI control method from the beginning, the time error oscillates between positive and negative and it takes time until the time error falls within the allowable range. There was a problem that it took.

  On the other hand, if the slave terminal clock correction method according to the first embodiment is used, as shown in FIG. 7, after receiving the first synchronization message and follow message from the master terminal, It is possible to reduce the time error between the time and the slave time within an allowable range. In addition, since the PI control is excellent for regular correction, the correction of the time error may be continued by the PI control after the time error is within the allowable range.

  Note that, as described in the above formulas (3) and (4), in the first embodiment, the error calculation unit 305 shown in FIG. 2 is connected to the slave terminal 10 from the master terminal 1 in the network 5 shown in FIG. The time error E is calculated on the assumption that there is no transmission line delay Dms. Here, the bidirectional transmission path delay D in the network 5 can be calculated based on the master time and the slave time, as will be described later. However, when the time error E between the master time and the slave time is large, the transmission line delay D calculated based on the master time and the slave time can also include a large error. Therefore, the accuracy of the value N of the correction variable calculated by the above equation (10) can be improved by calculating the time error E with the transmission line delay D set to 0.

  5, even if the slave terminal 10 shown in FIG. 2 receives a new synchronization message during the adjustment time τ, the synchronization message received during the adjustment time τ is the slave clock 201. It is not used for correction. However, it is possible to correct the slave clock 201 at an accurate timing by using a new synchronization message received when the process returns to step S101 after the adjustment time τ has elapsed, for the subsequent correction of the slave clock 201. Become.

(Modification of the first embodiment)
In FIG. 5, the example in which the synchronization message received during the adjustment time τ is not used for the correction of the slave clock 201 shown in FIG. 2 in steps S108 and S109. On the other hand, as shown in FIG. 8, a step of determining whether or not a “synchronization message” has been received may be inserted between step S108 and step S109. In this case, when the slave terminal 10 receives a new “synchronization message” during the adjustment time τ, the process returns to step S102, and calculation of a new correction variable N is started without waiting for the adjustment time τ to elapse.

(Second Embodiment)
In the first embodiment, the error calculation unit 305 shown in FIG. 2 assumes that there is no transmission path delay Dms from the master terminal 1 to the slave terminal 10 in the network 5 shown in FIG. Thus, the example of calculating the time error E has been described. However, if the network 5 is a small scale such as a local area network (LAN) and the number of connection nodes is limited, the value of the bidirectional transmission line delay D in the network 5 is, for example, 20 There are cases where it is constant in about microseconds and there is little fluctuation. Therefore, in the slave terminal 10 according to the second embodiment shown in FIG. 2, for example, a value of a predetermined transmission line delay D input by an operator or the like is stored in the nonvolatile parameter storage device 350. In this case, the error calculation unit 305 shown in FIG. 2 substitutes the value of the transmission line delay D read from the parameter storage device 350 for the variable of the transmission line delay Dms in the above equation (3), and calculates the time error E. calculate. Since the other components of the slave terminal 10 according to the second embodiment are the same as those of the slave terminal 10 according to the first embodiment, description thereof will be omitted.

(Third embodiment)
As illustrated in FIG. 9, the CPU 300 of the slave terminal 10 according to the third embodiment further includes a delay acquisition unit 309. The delay acquisition unit 309 transmits the “request message” illustrated in FIG. 10 to the master terminal 1 illustrated in FIG. Also, the delay acquisition unit 309 illustrated in FIG. 9 acquires from the slave clock 201 the uncorrected slave time Ts_t1 when the port 301 transmits the “request message”. The master terminal 1 shown in FIG. 1 acquires the master time Tm_r1 when the port receives the “request message” from the master clock. Furthermore, the master terminal 1 instructs the slave terminal 10 to transmit a “response message” with the master time Tm_r1.

The transmission line delay Dms from the master terminal 1 to the slave terminal 10 in the network 1 is given by the following equation (13) by modifying the above equation (3).
Dms = (Ts_r1 -E)-Tm_t1 (13)
Further, the transmission line delay Dsm from the slave terminal 10 to the master terminal 1 in the network 1 is the uncorrected slave time Ts_t1, the master time Tm_r1, and the time error E acquired using the “request message” and the “response message”. And is given by the following equation (14).
Dsm = Tm_r1-(Ts_t1-E) (14)
When the transmission line delay D in the network 1 can be considered to be symmetric, the delay acquisition unit 309 illustrated in FIG. 9 can perform transmission line delay Dms from the master terminal 1 to the slave terminal 10 as illustrated in (15) below. The average value of the transmission line delay Dsm from the slave terminal 10 to the master terminal 1 is calculated as the transmission line delay D in the network 1.
D = (Dms + Dsm) / 2
= [(Ts_r1 -E)-Tm_t1 + Tm_r1-(Ts_t1-E)] / 2
= (Ts_r1-Tm_t1 + Tm_r1-Ts_t1) / 2 ... (15)
Therefore, in the third embodiment, the error calculation unit 305 calculates the time error E by substituting the value of the transmission path delay D calculated by the delay acquisition unit 309 into the variable Dms in the above equation (3). To do. Since the other components of the slave terminal 10 according to the third embodiment are the same as those of the slave terminal 10 according to the first embodiment, description thereof will be omitted.

(Other embodiments)
As described above, the present invention has been described according to the embodiment. However, it should not be understood that the description and drawings constituting a part of this disclosure limit the present invention. From this disclosure, various alternative embodiments, embodiments, and operation techniques should be apparent to those skilled in the art.

  For example, the master terminal 1 shown in FIG. 1 may have the same sub-counter and main counter as the slave terminal 10. The slave terminal 10 synchronized with the master terminal 1 may function as a master terminal for the other slave terminals 14 and 18. In FIG. 2, the slave clock 201 and the like are expressed as modules of the CPU 300, but the slave clock 201 and the like may be independent hardware.

  As shown in FIG. 11, the value of the correction variable N calculated based on the first synchronization message and the follow message is continued even after the adjustment time τ has elapsed and the time error E is within the allowable range. In some cases, the time error E may increase again. In this case, after the adjustment time τ elapses, the value of the correction variable N is reset to 0, or after the time error E is within the allowable range, the correction is made by the PI control method or the like. Good. However, when the tick rate with the master clock and the tick rate with the slave clock are sufficiently equal, the time error E does not increase again. Thus, it should be understood that the present invention includes various embodiments and the like not described herein.

DESCRIPTION OF SYMBOLS 1 Master terminal 5 Network 10 Slave terminal 201 Slave clock 301 Port 302 Sub counter 303 Main counter 304 Time calculation part 305 Error calculation part 306 Correction value calculation part 307 Clock correction part

Claims (7)

A slave terminal that can be connected to a master terminal via a transmission line,
A sub-counter that calculates a sub-count by adding a tick rate for each time interval;
A main counter that increments the main count when the subcount reaches a threshold;
An error calculating unit that calculates a time error between the uncorrected slave time calculated from the main count when the uncorrected tick rate is used and the master time of the master terminal;
Correction value calculation for calculating a value of the correction variable such that a correction time difference between the correction slave time calculated from the main count and the uncorrected slave time becomes equal to the time error when a correction variable is added to the tick rate And
A clock correction unit for adding the value of the calculated correction variable to the tick rate;
A slave terminal comprising:   The slave terminal according to claim 1, wherein the error calculation unit calculates the time error on the assumption that there is no transmission line delay in the transmission line.   The slave terminal according to claim 1, further comprising a transmission path delay storage device that stores a transmission path delay value acquired in advance in the transmission path.   The slave terminal according to claim 3, wherein the error calculation unit calculates the time error using the value of the transmission line delay. Adding a tick rate for each time interval to calculate a subcount,
Incrementing the main count when the subcount reaches a threshold;
Obtaining the master time from the master terminal via the transmission line;
Calculating an uncorrected slave time calculated from the main count when the uncorrected tick rate is used, and a time error between the master time;
Calculating a value of the correction variable such that a correction time difference between the correction slave time calculated from the main count and the uncorrected slave time is equal to the time error when a correction variable is added to the tick rate;
Adding the value of the calculated correction variable to the tick rate;
A method for correcting the clock of a slave terminal, including:   6. The slave terminal clock correction method according to claim 5, wherein the time error is calculated on the assumption that there is no transmission line delay in the transmission line.   6. The slave terminal clock correction method according to claim 5, wherein the time error is calculated using a transmission path delay value in the transmission path acquired in advance.