Multimeter ICs

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This page lists some information about ICs commonly used in various multimeters (DMMs).

Overview

Many multimeters use a special-purpose multimeter IC internally. This table lists those chips, as they're often directly responsible for the protocol and data format of the PC logging functionality of a multimeter.

Vendor Device Builtin PC interface Comments
Cyrustek ES51922 RS232, TX only, 19230 baud, 7o1 Data is sent via the SDO pin. Data logging can be en/disabled via RS232 pin. Some 3rd-party parsing utilities are listed below.
Fortune Semiconductor FS9721_LP3 RS232, TX only, 2400 baud, 8n1 Data is sent via the TXD pin. Data logging can be en/disabled via ENTX pin.
Fortune Semiconductor FS9922_DMM4 RS232, TX only, 2400 baud, 8n1 (?) Data is sent via the TXD pin. Data logging can be en/disabled via TXEN pin (?) and the REL/RS232 pin (?).
Intersil ICL7106 ?
Intersil ICL7136 ?
Intersil ICL7139/ICL7149 none (?)
MASTECH M343-01 ?
Maxim MAX130/131 ?
Maxim MAX133/134 ?
Metex KS57C2016 ? Possibly a relabel'd Samsung KS57C2016 4-bit microcontroller.
New Japan Radio NJU9207 none

Cyrustek ES51922

The Cyrustek ES51922 is an all-in-one multimeter chip. The data protocol is mostly described in the datasheet.

It is used in many multimeters, e.g. the UNI-T UT61E or the Wintex TD2200.

Resources

Fortune Semiconductor FS9721_LP3

FS9721_LP3 on Voltcraft VC-820.
FS9721_LP3 (?) on TekPower TP4000ZC.

Protocol

The chip periodically sends 14-byte packets at 2400 baud, 8n1. The upper nibble of each byte indicates the byte number. The payload is composed of the lower nibbles, and is a 1-1 mapping of the LCD segments. The downside to this protocol structure is that transmission errors in the LSB nibbles cannot be detected. There is no checksum or CRC in the packet. The 4 user-defined bits can have different meaning on different multimeters using this IC.

Packet structure:

Byte Bits 7-4 Bit 3 Bit 2 Bit 1 Bit 0
0 0x1 AC DC Auto RS232
1 0x2 Negative 1A 1B 1C
2 0x3 1D 1E 1F 1G
3 0x4 DP1 2A 2B 2C
4 0x5 2D 2E 2F 2G
5 0x6 DP2 3A 3B 3C
6 0x7 3D 3E 3F 3G
7 0x8 DP3 4A 4B 4C
8 0x9 4D 4E 4F 4G
9 0xa u n k Diode
10 0xb m % M Beep
11 0xc Farads Ohms Rel Hold
12 0xd A V Hz Low battery
13 0xe User bit 3 User bit 2 User bit 1 User bit 0

Segment lettering:

  C  
   
B G
   
  F  
   
A E
   
  D  

Fortune Semiconductor FS9721B

TODO.

Fortune Semiconductor FS9922-DMM3

TODO.

Fortune Semiconductor FS9922-DMM4

FS9922-DMM4 in a UNI-T UT61D.

The Fortune Semiconductor FS9922-DMM4 (datasheet) is a widely used 6000-count auto-ranging DMM chip: it takes input from the various controls on the front panel, drives the LCD display, and can communicate its readings via a serial port.

Protocol

TODO

Metex 14-byte ASCII

MASTECH M343-01 in a MASTECH MAS345.
Metex KS57C2016 in a RadioShack 22-168.

Protocol

Various multimeters from Metex (and rebadged ones) use this common 14-byte packet based ASCII protocol. The actual multimeter ICs in the devices have different names (e.g. "MASTECH M343-01" or "Metex KS57C2016") but they all use the same common protocol format.

The serial port settings can vary from device to device, e.g. 600/1200 baud, 7n2, RTS low, DTR high.

Every time the host sends a (any?) character to the DMM (most software packages send a D/0x44), it returns 14 ASCII bytes containing the measurement mode, value, and units. Even with continuous packet requests, the IC will only send a packet when a new measurement is available.

There is no additional other information about the multimeter status returned in the packets (e.g. no low-battery bit, no relative/min/max/hold bits, and so on).

Packet structure

Byte(s) Description
1-2 Measuring mode (DC, AC, OH, CA, TE, DI, ...)
3 Always a space (?)
4 Sign ('-' or ' ', i.e. a minus or a space)
5-9 Decimal point and current measurement value (Examples: 10.00, 0L, 3.999)
10-13 Unit (Examples: V, mV, A, mA, kOhm, MOhm, nF, ...)
14 Carriage return (13/0x0d)

Example packets

01 02 03 04 05 06 07 08 09 10 11 12 13 14
D C - 0 0 0 . 0 V \r
A C 0 0 . 0 0 A \r
C A 0 . 0 7 1 n F \r
O H O . L M O h m \r

Alternative Protocol

Some older devices like the Metex M-3650CR use a slightly different protocol:

Packets

Range 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Remarks
All       SPC CR
D/Beep       O L m V No contact
 1 2 3 4 m V Contakt/forward voltage
200 Ω     O . L O h m No contakt/overload (invalid value)
 1  2 3 . 4 O h m Value
2 kΩ    . O L k O h m No contakt/overload (invalid value)
 1  . 2 3 4 k O h m
20 kΩ     . O L k O h m No contakt/overload (invalid value)
 1  2 . 3 4 k O h m
200 kΩ     O . L k O h m No contakt/overload (invalid value)
 1  2 3 . 4 k O h m
2 MΩ    . O L M O h m No contakt/overload (invalid value)
 1  . 2 3 4 M O h m
20 MΩ     . O L M O h m No contakt/overload (invalid value)
 1  2 . 3 4 M O h m
200 mV = D C   O . L m V Overload
D C -  1  2 3 . 4 m V
2 V = D  C  . O L V Overload
D C -  1  . 2 3 4 V
20 V = D C     . O L V Overload
D C  1  2 . 3 4 V
200 V = D C     O . L V Overload (guessed, not tried ;-))
D C -  1  2 3 . 4 V
1000 V = D C     O L V Overload (geraten, nicht ausprobiert...)
D C -  1  2 3 4 V
200 mV ≈ A  C   O . L m V Overload
A C  1  2 3 . 4 m V
2 V ≈ A  C  . O L V Overload
A C  1  . 2 3 4 V
20 V ≈ A  C     . O L V Overload
A C  1  2 . 3 4 V
200 V ≈ A  C     O . L V Overload
A C  1  2 3 . 4 V
750 V ≈ A  C     O L V Overload (guessed, not tried ;-))
A C  1  2 3 4 V
2 mA = D  C  . O L m A Overload, over measurement range
D  C - 1  . 2 3 4 m A
200 mA = D  C  O . L m A Overload, over measurement range
D  C - 1  2 3 . 4 m A
20 A = D  C O  . L A Overload, over measurement range (guessed, not tried)
D  C - 1  2 . 3 4 A
2 mA ≈ A  C  . O L m A Overload, over measurement range
A  C 1  . 2 3 4 m A
200 mA ≈ A  C  O . L m A Overload, over measurement range
A  C 1  2 3 . 4 m A
20 A ≈ A  C O  . L A Overload, over measurement range (guessed, not tried)
A  C 1  2 . 3 4 A
hFE 1 2 3  4
Logics  R E A D Y Start status
 H i High.
 L o Low.
 F L O A T Undefined state.
2000 pF   O L p F Overload (capacity larger than measurement range)
  1 2 3 4 p F
200 nF   O . L n F Overload (capacity larger than measurement range)
 1 2 3 . 4 n F
20 μF   . O L u F Overload (capacity larger than measurement range)
 1 2 . 2 4 u F
20 kHz  0 1 . 2 3 k H z
200 kHz  0 1 2 . 3 k H z

"-" means "-" for negative value, space for positive value.

Commands

Command
D Send data currently displayed
M Send data stored in memory (up to 5 data sets)
C Clear data memory
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