PC serial port buffer

Summary of circuit features

• Brief description of operation: Buffer to run RS-232 data to longer distanced as normally
• Circuit protection: No special protection circuits used
• Circuit complexity: Very simple two transistor buffer circuit
• Circuit performance: Worked nicely in one special application, doubled the line throughput
• Availability of components: Widely available components at the time when the circuit was built
• Design testing: Circuit was in constatant use by my friend for over a year
• Applications: Maximizing RS-232 line throughput on long cable runs
• Power supply: +-12V DC power supply 80 mA
• Estimated component cost: Few dollars
• Safety considerations: No special safety considerations

Circuit description

This is a simple serial port buffer circuit I designed for a friend to speed up his SLIP connection in campus computer network "TRINET" of Helsinki University of Technology. The problem in the network was that the RS232 commections from rooms to terminal server were long and made of bad quality wiring.

The circuit is a simple buffer which adds more driving capacity to PC serial port for the signal to go succesfully from PC computer to terminal server (other direction had no problems). The computer is connected to connector CON1 and the buffered output is available ar CON2. With this circuit the speed of RS232 connection to termial server could be succesfully raised for 9600 bps to 38400 bps.

The circuit is basically a two transistor buffer consistong of transistors Q1 and Q2 which can drive up to 1A current pulses, but the maximum putput current of the circuit is limited by resistor R2. Value R2 was experimentally selected by testing resistor values in range of 22 ohm to 270 ohm and value 140 ohm gave best results (it provides quite good impedance matching to cable used). It is a good idea to use at least 1W resistor in place of R2 to make sure that it does not overheat in output short circuit situation (RS232 devices must withstand that to meet the standard).

The circuit was designed to be a compact box which is powered through D25 connector as some commercial RS232 buffer circuits. The idea is to feed the power to the buffer unit through serial port voltage test pins 9 and 10. The power was taken from an external power supply (cheap universal wall transformer) and wired to the D25 connector by modifying the cable connected between computer and the buffer circuit. The circuit in this configuration takes maximally continuous current of about 100 mA.