protocol

Analysis of UT61 communications

Everything described in this document is implemented in m_ut61.h, so
that file can be referred to as a working example.

USB cable

Start logging:
(This is a 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. I have identified two
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.

RS232 cable

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.

This is how the Windows application sets things up:
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

Message format

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.