Hello to anyone who cares about this Radio Shack Digital Multimeter cat# 220-0812. I wrote an interface for myself, that I'd like to share with you. The requirements are unix and a copy of gambas, and you get the full source code. Note that, I found, that, for whatever reason, the instrument sends wrong checksums every once in a while - you can see the number of resets. I know it's the instrument's or serial connection's fault, because when displaying 0.F MOhms overrange continuously, I know nothing changes, yet the correct 9A checksum from the device changes to
12 and 0, and other values, about every dozen 9-byte packet. This also messes up the byte-order. Doing a port reset, disconnect reconnect resyncs things. When there is a real resistance measured there are almost no resets. Another sensitive knob position with lots of resets over checksum errors is the temperature-mV one, when values widely fluctuate. Still, with a capability of 3 readings per second, you can just ignore any off-values and keep the rest, the device still functions very well, for the price. You can guess some costs were saved. Instead of summing checksum, they could have provided a CRC8, and made the packet 10-byte with an STX,ETX or LF char signifying end or beginning of packet, so there is never an issue trying to figure out which is the starting byte, and there would be no need for resets. Also some hardware handshake option would have been nice, because DTR/DSR and RTS/CTS are just dummy looped back to each other. But still a fun little device, and excellent cost, considering it's from a brick-store-in-your-neighbourhood retailer. Grab the program at
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and you may view a screenshot (377KB png image) at
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