RadioShack 22-812

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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

    ...

  • 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. 

                               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

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