I found this description of the protocol on internet some time ago, can’t find it again, if someone knows please send me a link.
HID Set Report request
CT 21 09 00 03 00 00 00 00
To read from the device listen to interrupt transfers from endpoint 82. There you’ll find 8 byte blobs roughly every 10 ms. There is two identified types (They are likely described in the USB HID spec), identified by their first byte:
f0 - No data available, the rest of the bytes are 0 f1 - Here, have a character! Second byte is the character.
If you assemble all the contiguos characters, you will end up with a 14-character message as described below.
The RS232 cable uses 2400 baud with 8 bit characters, no parity and 1 stop bit. To activate it, the RTS line needs to be cleared and the DTR line set. As the data packets all end with CRLF, you can use line-oriented reading from the port. SET_BAUD_RATE 2400 CLR_RTS SET_DTR SET_LINE_CONTROL 8n1 SET_CHAR EOF:0 ERR:0 BRK:0 EVT:0 XON:11 XOFF:13 SET_HANDFLOW Shake:1 Replace:0 XonLimit:512 XoffLimit:1 PURGE TXABORT RXABORT TXCLEAR RXCLEAR SET_QUEUE_SIZE InSize: 2048 OutSize: 2048 SET_TIMEOUTS RI:1 RM:0 RC:1 WM:0 WC:0
0: +/- 1-4: Digits (but see below) 5: Space 6: Precision 7-8: Flags 9: Prefix and special flags 10: Unit 11: Relative measurement integer 12-13: CRLF
The four digits are the measured value with the decimal point removed.
However, if the meter is overloading, the value “?0:?” is reported.
The precision is a single ASCII digit with the following meaning:
0: There is no decimal point 4: There is one decimal digit 1: There are three decimal digits 2: There are two decimal digits The seventh and eighth bytes are a bit field. I have observed these bits: 0x0001 Bar graph visibility 0x0002 Data hold 0x0004 Rel 0x0008 AC 0x0010 DC 0x0020 Autorange 0x0200 Nano prefix 0x1000 Min 0x2000 Max Byte 9 consists of two 4-bit fields, prefix and special flags. The prefix has the following meaning: 0x00 No prefix (or nano prefix, as per above) 0x10 Mega (M) 0x20 Kilo (k) 0x40 Milli (m) 0x80 Micro (µ) The special flags are: 0x00 Nothing special 0x02 % 0x04 Diode 0x08 Buzzer The unit byte reads as follows: 0x00 Percent 0x04 Fahrad 0x08 Hz 0x20 Ohm 0x40 Ampere 0x80 Volt
Byte 11 contains a 7-bit signed integer corresponding to the measured value in 64ths of the current range.