In my previous post, I did a review and teardown of a cheap BSide ACM03 Plus clamp meter and concluded that it was a surprisingly good meter despite a few minor annoyances.

After I did the review, I decided to do some investigation to find out what chip the multimeter used and whether there were any hidden features we could find.

Like most cheap multimeters, the ACM03 uses a COB (Chip on Board) design. Since there’s no marking on the chip it was a bit more difficult to figure out exactly what it was.

Luckily, there are not that many 100 pin 4000 counts multimeter chips. From the pins where the crystal is connected and the unused pins I was able to trace the chip back to Fortune Semiconductor’s FS9721 LP3 4000 counts autorange DMM IC. And by measuring the crystal frequency, I also confirmed that it was using a 4MHz crystal as in the reference design.

If you read through the datasheet, you will see that FS9721 LP3 also supports RS232 communication. This is achieved by pulling pin 84 (ENTX) to negative VSS (-1.5V, pin 44). The RS232 serial output (TXD) is located at pin 64.

Unfortunately although these pads are in place, they do not seem to be connected to the die. Presumably it saves a bit of manufacturing cost by eliminating unused bond wires. Or it could have been deliberate so you would have to fork out more for a more expensive model if you need the functionality. Either way, this is really a shame as you wouldn’t be able to modify the meter for the serial output like I did with my DTM0660L based ennoLogic meter.

That said, I am sure there are other meters out there using the same FS9721 LP3 chip and you may be able to enable the RS232 output by doing the modifications mentioned above.

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