The differences between them are:
- RS232 Interface
- True RMS
Turns out you can enable the RS232 interface by shunting pin 84 to GND, you will then obtain the data through pin 64 which is TX.
Originally pin 64 would drive the base of a PNP (2n3906) which in turn would drive an IR LED, this LED provides optical isolation — Typically there would either be an external plug-in module with a photo-transistor or it would have the module built-in, with the connector exposed to the user.
My suggested modification would only require a small hole at the back of the enclosure to expose the LED, you could then interface with it rather easily.
True-RMS cannot be “unlocked” easily since this feature requires an extra IC (AD737 or similar) which obviously isn’t populated on the C version. Other components around this IC are required as well.
I’m not sure but perhaps the quiescent current would increase if you have the RS232 mode enabled, it may be worth testing.
For a protocol specification you may refer to the FS9721 LP3 datasheet, even though it’s in Chinese you should be able to comprehend most of it. Scroll near the bottom to find the RS232 section.
Either way, if you have the UT60C and you wish you had data-logging capabilities, give this a go — You don’t have to make any permanent changes if you only intend to test it.