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.
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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.