JPH02161813A - Rs232c line driver ic - Google Patents

Abstract

PURPOSE:To prevent the malfunctions by keeping forcibly the output of an RS232C line driver IC at a prescribed level after detecting a fact that the power voltage is kept at a low level less than a prescribed level. CONSTITUTION:The power voltage rises up to a prescribed level (about 5V) at a V<+> terminal 101. Thus the voltage of a transistor TR Q103 is equal to 0.7V according to the dividing ratio between an R104 and an R105. Then a TR Q103 is turned on and both TR Q104 and Q105 are turned off. At the same time, a line driver IC starts its operation with the voltage applied to an input terminal 102. When the power voltage of the terminal 101 drops less than a prescribed level, the TR Q103 is turned off and both TR Q104 and Q105 are turned on with the output set at the OFF (low) side. As a result, the output is always controlled to the OFF (low) side when the voltage is kept at a low level at the rise and the fall of a power supply. Then no malfunction is caused.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides digital data transmission (j′
This invention relates to a control circuit for a line driver IC that complies with the RS232C standard among the Naubra type.

[Conventional technology]

Conventionally, as a line driver IC conforming to this type of RS 232 C standard, the most common line driver IC has the circuit system shown in FIG.

This driver IC operates between the piezo voltage V+ applied to the terminal 301 and the negative voltage ■- applied to the terminal 304, and includes a common base PNPI transistor Q301, a phase splitter Q303, a totem pole connected NPN transistor Q304, Q305, diode D301~D
308, resistors R301 to R308, an input terminal 302, and an output terminal 305.

In this IC, the voltage applied to the input terminal 302 is 1.2
When the voltage is low to V, the PNP transistor Q301° phase splitter Q303 and the NPN transistor Q305
Then, the output terminal 305 becomes high level, and conversely, 1 and 2
■Higher transistors Q301, Q303. Q305
is turned on, and the output terminal T305 becomes low level.

FIG. 5 is a schematic diagram of a system for remotely turning on the power of a terminal device from a host computer via a modem using the IC shown in FIG. 3, and FIG. 6 is a time chart thereof.

This system includes a host computer 501, a host modem 504, a terminal modem 508, and a terminal device 5.
11, and signals are transmitted between them. When the output of the transmission request output terminal line driver 502 provided at the output section of the host computer 501 becomes high level (FIG. 6(a)), this signal is sent as a transmission request in the host modem 504 via the electric cable 503. The signal is input to the input terminal line receiver 505, and the line receiver 505 sends out an inverted output as shown in FIG. 6(b). The frequency modulation/demodulation circuit 506 generates an analog signal shown in FIG.
sent to. When the frequency modulation/demodulation circuit 509 provided in the terminal side modem 508 receives the analog carrier, it turns on (high level) the output of the carrier detection output terminal line driver 510 (FIG. 6(d)). Carrier detection input terminal line driver 512 in terminal device 511
detects this via the electric cable 16, and as shown in FIG.
), the output of the flip-flop 513 is inverted (FIG. 6(r))b, and this inverted output turns on the electric switch 515 via the buffer 514. The edge trigger method is adopted in the remote power-on system of the terminal device 511 based on this transmission request, and when turning off the power of the terminal device 511 that has been turned on, the host computer 501
This is done by sending a send command from .

[Problem to be solved by the invention]

The above-mentioned line driver IC based on the conventional RS232C standard Q will malfunction because the output signal level will be high regardless of the input signal level at low voltages when the power supply voltage rises and falls. The disadvantage is that this occurs. This is because, for example, even if the level of the signal input to the input terminal 302 is set to high level and the output is set to low level, the internal bias is not normal and the transistor Q301. Q303. Q305 is not turned on, and the level of the output signal rises to the high level side due to the output of the mitter follower Q304. This relationship is illustrated in FIG. 4, and when the positive power supply voltage rises, the output voltage rises almost following the rise of the positive power supply voltage.

If such a rise in output voltage occurs immediately after power is turned on, in the system that remotely turns on the power of the terminal device described above, when the power of the host computer 501 is turned on, the high level output is sent to the line of the modem 504 on the host side. Since the receiver 5.5 detects this and sends out the analog carrier, a serious problem arises in that all the terminal devices 511 connected to the host computer are turned on unconditionally. It is necessary to sequentially send an OFF command to turn off the power of terminal devices that are not communicating. Further, a similar phenomenon occurs when powering down the host side, and the power of a plurality of terminal devices 511 that have been previously disconnected by an off command is turned on all at once. One way to prevent such malfunctions is to connect a mechanical relay to the output of the line driver IC and control the transmission of the IC output by controlling this relay, but in this case, the board area and power consumption increase. death,
There is a problem that efficiency deteriorates.

[Means to solve the problem]

The RS232C line driver 1c of the present invention is characterized in that, when the power supply voltage is a low voltage below a predetermined value, it has means for detecting this and forcibly holding the output of the RS232C line driver IC at a predetermined level. do.

[Effect]

Therefore, even in the low voltage region when the power supply rises and falls, the output voltage of the IC is maintained at a low level, for example, and the above-mentioned system malfunction does not occur.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the RS232C line driver IC of the present invention.

In this embodiment, an output control circuit 200 is added to the conventional example shown in FIG.
The other configurations are the same.

However, the reference numbers for each transistor, diode, and resistor are 1 to clarify that they are embodiments of the present invention.
The numbers are in the 00s. The output control circuit 200 has N
PNI-transistor Q103. Q104, PNP transistor Q105, and resistors R104, R106, and R112.

Next, the operation will be explained.

■If the power supply voltage of + terminal 101 is higher than O67■,
Transistor Q104. Q105 turns on, supplies base current to transistor Q106, and output terminal 105 becomes off (low). Next, ■ When the power supply voltage of the + terminal 101 increases and reaches the specified voltage (approximately 5Vl), R104 and R1
Due to the division ratio of 05, the base voltage of Q103 becomes 0.7V, 1~ transistor Q103 is turned on, 1~ transistor Q104 and Q105 are turned off, and line driver I
C operates based on the voltage applied to the input terminal 102. ■When the power supply voltage of the + terminal 101 becomes lower than the specified voltage, Q103 is turned off, and transistor Q104. Q
105 is turned on, and the output is turned off (low) side. Therefore, the output is always controlled to the off (low) side when the power supply rises and falls at low voltage.

FIG. 2 is a circuit diagram of another embodiment of the RS232C driver IC of the present invention.

The line driver of the present invention employs a charge pump type D
It consists of a C-D Cml inverter, a driver circuit, and a power-on reseller 1- circuit, the DC-DC converter supplies power, and the power-on reseller 1- circuit supplies the driver output of the driver circuit when the power supply voltage rises. It functions to fix the level of the terminal 214 to a low level.

The driver circuit includes a driver input comparator 206;
driver output control input" comparator 211, inverters 212, 209, and gate 207. A NOR gate 208, a first latch consisting of MOS transistors Q201 to Q204, and MOS transistors 0205 to 0
a second latch consisting of 20B, a non-inverting driver output buffer 213, a driver input terminal 205, a driver output control tlE terminal 210, and a driver output terminal 214.
It is made up of.

The power-on reset circuit also includes MOS l-transistors Q209, Q213, and MOS t-transistors Q214 to Q220. A comparator consisting of resistors R201 and R202, and a MOS transistor Q221-
0224, and an output buffer consisting of inverters 215-217.

In addition, the charge pump type DC-DC:1 inverter 202 has a Vce terminal 201 with +5V applied to it.
-tlOV is generated at the DDE termination 204, and -10V is generated at the Vss terminal 203. DC-DC2] Converter Voo
, Vss voltages are generated by charging and discharging the terminal ( ), so that a swell occurs when the voltage of Vce is turned on and off.

That is, after inputting 5■ to the Vce terminal 201, an I voltage of several hundred milliseconds occurs until a constant voltage is generated at the Vpo terminal 204 and the Vss terminal 203, and conversely, the power to the Vce terminal 201 is turned off. In this case, even if the Vce terminal 201 becomes OV, the VD[) terminal 204 and the V ss terminal 2
There is a delay of several hundred msec until the MH of 03 becomes OV.

Next, the operation will be explained.

First, ``5■'' is applied to the Vce terminal 201, and Vs
When the power is turned on, when the s terminal 203 has not reached the specified voltage (approximately -4.5 V'), the potential at point 13 is higher than the potential at point A in the power-on reset circuit, so transistor Q214 is turned on and 0216 is turned on. On, 02
18 is off, Q220 is turned on, the output of inverter 215 is high, the output of inverter 216 is low, the output of inverter 217 is high, the output of NOR gate 208 is low, and the output of inverter 209 is high. As a result, transistor Q203 is turned on, Q204 is turned off, and Q203 is turned on.
02 is on, Q201 is off, Q206 is on, Q205
is off, Q207 is on, and 0208 is off, and the driver output terminal 214 is controlled to the low (Vss potential) side. On the other hand, the Vss g terminal 203 is at the specified voltage of about -4
, 5V), the potential at point 8 becomes lower than the potential at point A, resulting in the opposite operation as described above, and
7 becomes low, so the driver output terminal 214
is controlled by the output of the Nant gate 207.

Further, the terminal 210 is a driver output control terminal, and when this terminal is set to low, the output of the comparator 211 becomes high and the output of the inverter 212 becomes low, so the signal input to the driver input terminal 205 is high or low. Regardless, the output of the Nant gate 207 becomes high, and therefore the driver output terminal 214 is controlled to the low side. Furthermore, when the driver output control O terminal 210 is set high, the output of the inverter 212 becomes high, so the Nant gate 207 is controlled by the output of the driver input comparator 206, and the line driver IC is operated by the voltage applied to the input terminal 205. Do this.

That is, the power-on reset is released, the output of the inverter 217 is low, and the driver output side t11 terminal 2
10 is high, if a high level signal is input to the driver input terminal 205, the driver output terminal 214 becomes low, and if a low signal is input to the driver input terminal 205, the driver output terminal 214 becomes high.

Next, when the power is turned off, when Vcc falls, VccI! While the gate connected to the B element 201 is operating, if the driver input terminal 205 is set high or the driver output control terminal 210 is set low, the driver output 214 is controlled to the low side. Also, Vcc
If the pressure has completely dropped, Vss terminal 203 and ■DD
If voltage remains at terminal 204, the inverter
The input and output of transistor Q203. Q204 is off, transistor Q20
1 drain (0 points), and Q202 drain (D
point) is high level (VD()-0201 or Q202
(threshold voltage of transistor Q205).
, Q206 is turned off, the drain of transistor Q207 (point E) and the drain of transistor 0208 (point 1) become low (Vss-1-0201 or the threshold voltage of Q202), and the driver output terminal 214 is controlled to the low side. .

As described above, in the second embodiment as well, the output is always controlled to be off (low side) at low voltages when the power supply rises and falls.

〔Effect of the invention〕

As explained above, the RS232C line driver IC of the present invention controls the driver output to the off (low) side at low voltages when the power supply rises and falls.
This has the effect of preventing malfunctions in a system in which the power of a terminal device is remotely turned on from a host computer via a modem.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of one embodiment of the RS232C line driver IC of the present invention, FIG. 2 is a circuit diagram of another embodiment of the RS232C line driver tC of the present invention, and FIG. 3 is a circuit diagram of a conventional RS232C line driver IC.
A circuit diagram of the S232C line driver IC. Figure 4 shows the output voltage characteristics of a conventional RS232C line driver IC at power up and power down. Figure 5 shows a system for remotely turning on the power of a terminal device from a host computer via a modem. , FIGS. 6(a) to 6(f) are diagrams showing output waveforms of each part in the system of FIG. 5. 101...V terminal, 102...Input terminal, 1
03...Ground terminal, 104...V'''''' terminal 105 River output terminal, 200
... Output control circuit, Q101 to Q108... Bipolar transistor, D
101~D108...Diode, R101~R11
2...Resistor, 201.Vcc (+5V power supply) terminal, 202...Charge bomb type DC-DC converter, 203-V ss terminal (-10V), 204.-V
oo terminal (+10 V), 205... Driver input terminal, 206... Driver input comparator, 207...
Nantes Gate, 208... Noah Gate, 209...
Inverter, 210... Driver output control terminal, 211... Driver output control input comparator, 21
2... Inverter, 213... Driver output buffer (non-inverting), 214
...Driver output shaft "1: Child, 215-217...
Inverter, to Q201/Q224...~10S transistor, R
201-R202...Resistor, 301...Sub terminal, 302...Input terminal, 30
3...Ground terminal, 304...■One terminal, 305
...output terminal, Q301 to Q305...bipolar 1 to transistor,
D301-0308...Diode, R301-R3
08...11 (resistance, 501...Boss 1~:1 viewer, 502...Transmission request output terminal line driver, 503...Electric cable, 504...Post side modem, 505...Transmission Saw terminal line receiver, 50
6...-Frequency modulation/demodulation circuit, 507... Analog line, 508... Terminal side modem, 509... Frequency modulation/demodulation circuit, 510... Gear rear detection output horn terminal line driver,
511...Terminal device, 512...Carrier detection input terminal line receiver, 5
13...Trigger flip-flop, 514...Buffer, 515...Terminal device power on/off switch, 16.
...Electric cable.

Claims (1)

[Claims] In the RS232C line driver IC, when the power supply voltage is a low voltage below a predetermined value, this is detected and the R
RS2 characterized by having means for forcibly holding the output of the S232C line driver IC at a predetermined level.
32C line driver IC.