Real RS232 for Laptop PCs

Many Metex DVMs feature a serial interface (RS232) which enables measured values to be copied to a PC for processing. Although this works just fine with most desktop PCs, problems may arise — as the author found out the hard way — when a laptop PC like the IBM Thinkpad 370C is hooked up to the Metex DVM. The cause of the problems is the limited voltage swing of just ±5 V on the 370C’s serial interface. This is simply not enough for the Metex DVM, which will appear ‘deaf’ to the laptop.

Please note that C4 is shown with the wrong polarity in the circuit diagram. On the printed circuit board overlay, the same goes for C1, C2 , C3 and the supply connections.