RadioShack 22-812
The Radioshack 22-812 is a 4000 counts, CAT II handheld digital multimeter with RS-232 connectivity. The 22-812 is no longer sold under this part number, and for a while, it appeared to have been discontinued. Radioshack does sell a visually identical multimeter, the 22-039. It is not yet clear if the Radioshack 22-039 is the same device as the 22-812.
This devices has a standard DB-9 connector at the top, and does not require a special adapter cable. The RS-232 transmitter is optically insulated from the rest of the device. The transmitter uses a separate PCB, integrated into the multimeter's housing.
Hardware
TODO
Photos
- Radioshack 22 812 front.jpg
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Device, angle
RS-232 connector
Cover removed, front
Cover removed, back
Battery locatiob
RS-232 transmitter
RS-232 module
RS-232 PCB, top
RS-232 PCB, bottom
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Protocol
The device periodically sends 9-byte packets at 4800 baud, 8n1. The packet includes a mode indicator, payload and checksum. the payload is a 1-1 mapping of the LCD segments.
Bit Byte 7 6 5 4 3 2 1 0 0 ---------------------- Mode ----------------------------- 1 Hz Ohms K M F A V m 2 u n dBm s % hFE REL MIN 3 4D 4C 4G 4B DP3 4E 4F 4A 4 3D 3C 3G 3B DP2 3E 3F 3A 5 2D 2C 2G 2B DP1 2E 2F 2A 6 1D 1C 1G 1B MAX 1E 1F 1A 7 Beep Diode Bat Hold - ~ RS232 Auto 8 -------------------- Checksum ---------------------------
The segment lettering is
|--A--| | | F B | | |--G--| | | E C | | |--D--|
The mode is given by the following table:
0 DC V 1 AC V 2 DC uA 3 DC mA 4 DC A 5 AC uA 6 AC mA 7 AC A 8 Ohm 9 Capacitance 10 Hz (dial set to Hz) 11 Hz (dial set to DCV) 12 Hz (dial set to mA/A) 13 Duty cycle (dial set to Hz) 14 Duty cycle (dial set to DCV) 15 Duty cycle (dial set to mA/A) 16 Pulse width (dial set to Hz) 17 Pulse width (dial set to DCV) 18 Pulse width (dial set to mA/A) 19 Diode 20 Continuity (beep) 21 hFE 22 LOGIC 23 dBm 24 (Unknown) 25 Temp
Resources
- rs22812.py project page (Python interface to Radio Shack 22-812 DMM)
- Radioshack 22-812 user manual