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.
Hardware
The micro-controller is an unidentifiable plastic blob.
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. The RS-232 module is powered from the serial port.
Photos
- Radioshack 22 812 front.jpg
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Device, angle
RS-232 connector
Cover removed, front
Cover removed, back
Battery location
Opened up
PCB removed
PCB, front
PCB, back
Buttons PCB
RS-232 module
RS-232 transmitter
RS-232 PCB, top
RS-232 PCB, bottom
Banana sockets connecting to PCB
Spare fuse
Chip
Calibration pots
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.
Packet structure
| 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 |
Segment lettering
The segment lettering is
|--A--| | | F B | | |--G--| | | E C | | |--D--|
Mode byte
The mode is given by the following table:
| Value | Meaning |
|---|---|
| 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