Multimeter ICs

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. Parsing utilities listed in Cyrustek ES51922.
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

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