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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CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor GREEN and GOLD members can exclusively download these files for free as part of their membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service.
Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.
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.