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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.
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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