JPS5917915B2 - signal isolation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a signal isolation circuit, and in particular provides a simple and compact multi-point input isolation circuit.

When transmitting signals that change digitally in a measurement system, a process control device, or the like, there are cases where it is desired to electrically separate the signal transmission path, that is, apply direct current insulation. For example, in process control equipment, when transmitting digital signals generated by the on/off of contacts, transistors, etc. on the process side, the presence or absence of voltage on the process side, etc., to the control equipment side, It is generally configured to provide direct current insulation between the process equipment side and the process equipment side. Relays, photocouplers, transformers, etc. are used as such insulation means, and various circuit systems have been proposed for using these. Particularly when performing direct current insulation on a large number of signal lines, it is important to have a simple structure that is inexpensive and space efficient, and the circuit system structure also places importance on these points. FIG. 1 diagrammatically shows a part of an insulated process control system, in which signals from equipment built into the process equipment, such as an A/D converter 2, are transmitted to a control device 1. In FIG. In that case, the digital signal from the A/D converter 2 is taken into the register 4 in the control device 1 via the isolation circuit 3, synchronized and sent as an internal signal to the arithmetic control section 5, where it is processed. and perform process control. Here, the insulation circuit 3 is for providing DC insulation between the A/D converter 2 and the control device 1. Conventional insulation methods used in the insulation circuit 3 include those shown in FIGS. 2A to 2C.

The one shown in FIG. 2A uses a so-called photocoupler 11 consisting of a light emitting diode, a phototransistor, and the like. Here, when the contact 12 is opened or closed, a current flows from the power source 13 through the resistor 14, and the light emitting diode 15 is turned on. As a result, the phototransistor 16 is turned on, and its output is read through the inverter 17 and a pulse R
The isolated output 0UT is taken out from the flip-flop 18 by reading it into the flip-flop 18 in synchronization with D. FIG. 2B shows an example in which a relay 21 is used instead of the photocoupler 11, and the current flowing through the coil 22 closes the contact 23, transmits a signal, and takes out an isolated output 0UT. FIG. 2C shows an example in which the opening/closing of the contact 12 is transmitted via a transformer 31. DC changes such as the opening and closing of the contact 12 cannot originally be transmitted by a transformer, so in this example, the circuit configuration for detecting the opening and closing of the contact 12 includes a diode 32 and a capacitor on the primary side of the transformer 31. 33 and further transformer 3
A pulse generator 34 and a diode 36 are provided on the secondary side of 2. With this configuration, a signal is obtained on the secondary side of the transformer 31 according to the opening and closing of the contact 12, and this signal is sent to the gate 3.
Isolated output 0U in synchronization with read pulse RD through 7
It can be read as T. Describing the drawbacks of these conventional methods in order, the method using a photocoupler shown in FIG.
Moreover, since the photocoupler itself is a relatively new element, there remain concerns about reliability regarding changes in transmission efficiency over time.

The method using a relay shown in FIG. 2B requires a power source 13 to detect the opening and closing of the contacts 12, and has the drawbacks of slow response speed and low reliability because it has mechanical contacts. . Although the transformer system shown in FIG. 3C does not require a power source for detecting the opening and closing of the contact 12, it does require the pulse generator 34 and its synchronization circuit, which has the drawback of complicating the circuit. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a signal isolation circuit having a simple configuration, small size, high reliability, and low cost.

The present invention provides an isolation transformer having a primary winding that receives an input signal, a secondary winding that is galvanically isolated from the primary winding, and that supplies an output from the secondary winding. and a memory circuit that holds the signal state of the output at the timing of the read signal, supplies a constant voltage to one end of the secondary winding via a resistor, and supplies the other end of the secondary winding of the transformer with a constant voltage. supplies a strobe signal, and while the strobe signal is being supplied, supplies the read signal to the storage circuit to read the signal state of the input signal applied to the primary winding, and reads the signal state of the input signal applied to the primary winding, It is characterized by being configured to take out an isolated output.

The invention will be explained in detail below with reference to the drawings.

FIG. 3 shows an example of the basic configuration of the insulated input circuit of the present invention.

In FIG. 3, a contact SW is directly connected to the primary side of the transformer T to form a closed circuit, so that the primary side of the transformer T is short-circuited or opened as the contact SW is turned on or off.
A diode D and an output terminal of a reading inverting gate G are connected to one end of the secondary side of the transformer T. The other end of the diode D is connected to a power supply Cc, and the inverting gate G is supplied with a strobe signal STB. The other end of the secondary side of the transformer T is connected to a resistor R and a D input terminal of a D-type flip-flop F for synchronizing read signals. Connect the other end of resistor R to power supply V
Connect to cc. The flip-flop F receives a read signal R.
D is also added, and isolated output 0UT is taken out from output terminal Q. The read signal RD and strobe signal STB are supplied synchronously, and the strobe signal STB
The secondary side of is driven, and the state of the secondary side impedance according to the on/off state of the contact SW on the primary side, that is, the signal level of the secondary side is taken into the flip-flop F by the read signal RD. do. 4A to 4D show the operation of the isolated input circuit shown in FIG. 3 when the contact SW is off.

At this time, since the primary side of the transformer T is open, the impedance on the secondary side of the transformer T becomes high. Therefore,
Even if the strobe signal STB comes in as shown in FIG. As shown in B, it becomes approximately Cc through the resistor R. At this time, when the read signal RD rises as shown in FIG. 4C, the high level D input is taken into the flip-flop F, and the flip-flop output 0UT becomes high level as shown in FIG. 4D. Furthermore, in FIG. 4D, the flip-flop output 0UT was at a low level before the read signal RD came in;
Of course, when the flip-flop output 0UT was at a high level before the RD signal was input, the output 0UT remains at a high level and does not change. As described above, the flip-flop F outputs the contact SW in synchronization with the read signal RD.
A high-level isolated output 0UT corresponding to the OFF state is obtained. Figure 5 A-D shows the third contact point when the contact SW is closed.
2 illustrates the operation of the illustrated circuit.

At this time, the primary side of the transformer T is short-circuited, so the impedance on the secondary side of the transformer T becomes extremely low.

Therefore, when the strobe signal STB rises as shown in FIG. 5A, the output of the inverting gate G is transmitted almost unchanged to the input terminal D of the flip-flop F, and the flip-flop FOD input goes low as shown in FIG. 5B. level.
Subsequently, as shown in FIG. 5C, when the read signal RD rises, the low level of the D input terminal is read into the flip-flop F, and the output 0UT of the flip-flop becomes a low level. In this way, an isolated output 0UT synchronized with the read signal RD is obtained at the output terminal Q of the flip-flop F.
As explained above with reference to FIG. 3, when the contact SW is open, the output of the flip-flop F becomes high level in synchronization with the read signal RD, and when the contact SW is closed, the output of the flip-flop F is the read signal RD.
It becomes low level in synchronization with.

Next, FIG. 6 shows an embodiment of the insulated input circuit of the present invention, which uses the basic circuit of the present invention shown in FIG. 3 to detect the opening and closing of a large number of contacts.

However, here, the contacts that open and close are not mechanical, but are shown using transistor switches as an example. n transistor switches T, l, T, 2
,...,T,. are respectively insulated transformers Tl, T2,
..., Tn, and transformers Tl, T2, ...,
One end of the secondary side of TO is commonly connected to the power supply potential Cc via resistors R,, R2, . . . , RO, and this one end is connected to the input terminals of n registers RG<7). moreover,
The other ends of the secondary sides of the transformers Tl, T2, . The operation of this example circuit is similar to that shown in FIG.
When the strobe signal STB rises to a high level, the transformers T,, T2, . , the level becomes low or high depending on whether the transistor switch Trl9Tr29″1°9Trn on the primary side of TO is on or off.These register inputs are taken into the register RG when the read signal RD is generated, and are stored in the register. Output 0UT1, 0UT2,...
, 0UTn are the corresponding transistor switches Trl, .
..., becomes a high level or a low level depending on whether Trn is on or off. Note that a gate G receives the strobe signal STB and drives the transformers Tl, T2, ..., TO.
The load of 1 is approximately equal to resistors Rl, R2, . . . and RO connected in parallel.

Therefore, the resistances Rl, R2,... and RO are set to several K.
If it is set to about Ω, about 10 to 20 isolation transformers can be driven using a normal TTL standard gate as the gate G1. That is, according to this example, a large number of insulation circuits can be driven with only one read gate G1.
The diode D, is for absorbing the rebound voltage generated from the transformer T, .about.TO when the output of the gate G rises.

In this example, by directly connecting one end of the diode D to the power supply potential Cc as shown, n transformers T
, ~Tn by one diode D1
can be absorbed. FIG. 7 shows another embodiment of the isolated input circuit according to the present invention, and like the embodiment shown in FIG. 6, this embodiment is connected to the primary sides of transformers Tl, T2, . Transistor switch Trl, T, 2
,..., detects whether Trn is on or off.

Generally, the contacts whose on and off states are detected (in this example, transistors Tr, Tr2, ..., Trn)
is active when it is on, and the detection signal obtained at this time, that is, the signal 0UT read from register RG.
1, 0UT2, . . . , 0UT0 are often set to be active at a high level, that is, positive logic. However, in the embodiment shown in FIG. 6, the signals 0UT, 0UT2, . . . , 0UTn read from the register RG are at a low level when the contact (transistor switch) is on. Therefore, in the embodiment shown in FIG. 7, in order to drive the transformers Tr,, Tr2, . . . , Trn by the transistor TR instead of the inverting gate G1 in FIG. Pnp each end of the secondary side
It is connected to the collector of the transistor TR, its emitter is connected to the power supply Vcc, and the strobe signal STB is applied to the gate. The other ends of the secondary sides of the transformers Trl to Trl are connected to resistors R11, R, 2, . . . , Rl, respectively. The ground terminals are connected to each input terminal of the register RG. A diode D2 for absorbing rebound voltage from the transformers T1 to Tn is inserted between the collector ground potential of the transistor TR. When the strobe signal STB falls, the transistor TR turns on, and when the transistor switches Tr, Tr2, . At the same time, it is read into register RG.

Conversely, transistor switches T, l, T, 2,..., T
When rn is off, the input of register RG remains at a low level even if strobe signal STB falls. In this way, the transistor switches T, l, Tr2, . . .
The register output corresponding to one of the Trns that is on becomes a high level, and the register output corresponding to one that is off becomes a low level. However, in this case, the resistance Rll, R, 2
,..., Rln is the signal source impedance when low level input to register RG, so register RG is connected to TT.
When configured with an L circuit, the values of these resistors Rl, .about.R, n are limited to several hundred ohms. According to the present invention, impedance changes on the primary side of a plurality of isolation transformers can be controlled with a simple configuration.
It can be detected on the secondary side of the transformer, which is galvanically isolated from the primary side of the transformer. In addition, only one circuit (gate G in Figure 6, transistor TR in Figure 7) to drive the secondary side of the transformer (gate G in Figure 6, transistor TR in Figure 7) is required, and only one diode is required to absorb the rebound voltage generated in the transformer. Therefore, the circuit configuration becomes simple when driving a large number of input isolation circuits. However, the present invention can be effectively applied to any application in which an input that changes the primary impedance of a transformer between high and low levels is detected in an isolated state on the secondary side of the transformer.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing part of a process control system having an isolated input circuit, Fig. 2 A, B, and C are circuit diagrams showing three examples of conventional isolated input circuit configurations, and Fig. 3 is a block diagram of a process control system having an isolated input circuit. A circuit diagram showing a basic configuration example of an isolated input circuit according to the invention, FIGS. 4A-D and 5A-D are signal waveform diagrams showing the operation of the circuit of FIG. 3, and FIGS. FIG. 7 is a circuit diagram showing the configuration of two embodiments of an isolated input circuit according to the present invention.

Claims (1)

[Claims] 1. An isolation transformer having a primary winding that receives an input signal and a secondary winding that is galvanically isolated from the primary winding, and that supplies an output from the secondary winding. and a memory circuit that stores the signal state of the output at the timing of the read signal, supplies a constant voltage to one end of the secondary winding via a resistor, and supplies the other end of the secondary winding of the transformer with a constant voltage. supplies a strobe signal, and while the strobe signal is being supplied, supplies the read signal to the storage circuit to read the signal state of the input signal applied to the primary winding, and reads the signal state of the input signal applied to the primary winding, A signal isolation circuit characterized by being configured to take out an isolated output. 2. The circuit according to claim 1, wherein a plurality of combinations of the isolation transformer and the storage circuit are provided, and a strobe signal is commonly supplied to all secondary windings of the transformer, The storage circuit is configured with a register having an input terminal to which a plurality of outputs from the transformer are individually supplied, and a plurality of inputs are supplied from the plurality of output terminals of the register to each of the transformers. A signal isolation circuit characterized in that a plurality of isolated outputs corresponding to each signal are individually taken out. 3. A signal isolation circuit according to claim 1, characterized in that a diode for absorbing rebound voltage from the secondary winding is connected to the secondary winding of the transformer. 4. A signal isolation circuit according to claim 2, wherein the strobe signal is commonly supplied to the secondary windings of the plurality of transformers. 5. The circuit according to claim 4, wherein a diode for absorbing rebound voltage from the secondary windings is commonly connected to the secondary windings of the plurality of transformers. signal isolation circuit.