Serial RS232 Booster Up to 100m


This circuit can run over relatively long distances (up to 100
metres), inexpensively, reliably, and suitable for speeds up to 2400
bauds. At the distances considered, the main expense is normally the cable, so here a readily available 60 ohm coaxial cable is used. Because of its relative immunity to noise, current drive is employed. In the line driver – figure 1 – transistor TI, diode D4, and resistors R3 and R4 form a current source that can be fed direct from a non-regulated supply of 8. . .I0 V. The transistor should be mounted on a heat sink.

Serial RS232 Booster Up to 100m

Serial RS232 Booster Up to 100m

The current level of 40 mA ensures an adequate input signal to the line receiver. Transistor T2 is a current switch that short-circuits the current source and the cable to earth of the input to the driver is logic high: only when that input is logic low, is the current of 40 mA fed into the cable. Diodes D2 and D3 protect the driver against noise emanating from the cable, while capacitor CI decouples the supply line. The line receiver is based on a type LM 311 comparator. Matching of the input
is effected by a wire link at a relevant tap of resistive divider
R5-R6- Rd Rs (in our case: 60 R). Resistors R9 and Rlo, and diode D5
protect the LM 311 against noise emanating from the cable. The sensitivity of the receiver
is set with Pi Resistor R14 provides some hysteresis. Pull-up resistor
RIS ensures that IC, provides at its pin 7 a TTL output signal that is
in phase with the input signal to the line driver. The
circuit is best calibrated with the aid of an oscilloscope once it has
been installed in its final position. The level of input to the receiver
is then compared with the voltage at the wiper of Pi The setting of P1
is optimum when the voltage at its wiper (wave form A in figure 3) is
exactly opposing the input voltage (wave form B in figure 3)


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