RadioShack 22-812

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RadioShack 22-812
Radioshack 22 812 front.png
Status supported
Source code serial-dmm
Counts 4000
IEC 61010-1 CAT II (600V)
Connectivity RS232 (standard DB9 connector)
Measurements voltage, temperature, current, resistance, capacitance, continuity, diode, logic, frequency, hFE, duty cycle, pulse width
Features autorange, data hold, min/max, dBmW (600Ω)

The RadioShack 22-812 is a 4000 counts, CAT II (600V) handheld digital multimeter with RS-232 connectivity.

It 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 RadioShack 22-039. It is not yet clear if the 22-039 is the same device as the 22-812.


The microcontroller 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.



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


Mode byte

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
23 dBm
24 (Unknown)
25 Temp