Telog Linecorder
RS232 signal amplifier

The circuit described is for use between a notebook PC and a Telog Linecorder when the notebook has, or is suspected of having, weak drive on the signal lines.

Circuit Description
A ground connection is provided from pin 5 or the case of P1. The usual ground signal is provided by pin 5 but it has been found on some notebooks computers that ground is provided only through the case of the RS232 socket. Resistors R1 & R2 limit any ground loop current that may exist between pin 5 and the case as pin 5 is often connected to ground on the motherboard.

The Tx signal is fed via R3 to the base of Q1. When in the 'mark' (rest) state D1 ensures the base of Q1 is not forced negative so as to start conduction. When the signal moves to the 'space' state Q1 conducts. This is turn forces Q2 on. This raises the bases of the output complimentary pair. The emitters, following the bases, pull the output high to the 'space' state.

When the input returns to the 'mark' state Q1 turns off, turning Q2 off, and R7 pulls the bases of the output pair negative, the output pair thus pulling the output to the 'mark' state.

Pin 5 is supplied from B1 so as to not drain the Tx pin of the PC port.

This circuit has one other feature in that no switch is required as when both ends are unplugged the circuit draws no current except leakage through the transistors (which is kept at a minimum by R4 and R6) which is less than the self discharge of the batteries!

Parts List

R1, R2 - 10 to 22ohms
R3-R7 - 10 kilo-ohms

D1 - 1N4148

Q1 - BC337-25
Q2 - BC327-25
Q3 - BC337-25
Q4 - BC327-25

B1 - 9v PP3
B2 - 9v PP3

If there are any questions please email