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	ES51978	RS232, TX only, 2400 baud, 7o1	Data is sent via the SDO pin. Data logging can be en/disabled via RS232 pin.
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.
Dream Tech International Ltd	DTM0660	RS232, TX only, 2400 baud, 8n1	Some people think this chip is a copy of the HY12P65. Protocol looks closely to the FS9721_LP3.
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 (?).
Hung Change Co Ltd	HCPD608	RS232, TX/RX, 9600 baud, 7n1	Used in Protek 608, Voltcraft VC608
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
WENS	WENS98A	RS232, TX only, 9600 baud, 8n1	used in Conrad Electronic / Voltcraft GDM 704

Cyrustek ES51978

The Cyrustek ES51978 is an all-in-one multimeter chip. The data protocol is well described in the datasheet.

It is used in many multimeters, e.g. the ISO-TECH IDM103N or the ISO-TECH IDM 98II.

See Multimeter ICs/Cyrustek ES519xx for a detailed comparison of the Cyrustek ES519xx IC series protocols.

Resources

• Datasheet
• 'Unlocking' multimeter RS232 output (info on hooking up a Cyrustek based multimeter to a computer)

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.

See Multimeter ICs/Cyrustek ES519xx for a detailed comparison of the Cyrustek ES519xx IC series protocols.

Resources

• Datasheet
• Multimeter data parsing utility complete implementation written in Python
• diyftw.de: Uni-Trend UT61E (UT-D04 linux treiber) (device info, Linux software using HIDAPI: ut61e-linux-sw-0.02.tar.gz)
• Steffen Vogel: UNI-TREND UT61E Digital Multimeter (device info, Linux software for serial port: dmmut61e-0.01.tar.gz)

Dream Tech International DTM0660

Very little information is available about this chip, even though it was put on the market in 2013. Most of the information found on the web is in Chinese. Searches suggest that some PeakTech, UNI-T, RadioShack, and Velleman DMMs use that chip.

DTM0660 in QFP package

Protocol

The protocol used by the DTM0660 looks similar to the one used by the FS9721_LP3 IC (see protocol description below), except that it adds one more byte to the packets (probably to accommodate supplemental symbols displayed on the LCD) and that the nibble order is reversed.

Protocol options

Interestingly the datasheet indicates that the chip is able to send 14-bytes packets and that the nibble order can be inverted by programming a special register in EEPROM, potentially allowing a full FS9721_LP3 compatibility. This also means that devices using the DTM0660 could have different protocol versions.

DTM0660 protocol options

Packet structure (can be affected by EEPROM registers)

Byte	Bits 7-4	Bit 3	Bit 2	Bit 1	Bit 0
0	0x1	RS232	Auto	DC	AC
1	0x2	1A	1F	1E	- (minus)
2	0x3	1B	1G	1C	1D
3	0x4	2A	2F	2E	DP1
4	0x5	2B	2G	2C	2D
5	0x6	3A	3F	3E	DP2
6	0x7	3B	3G	3C	3D
7	0x8	4A	4F	4E	DP3
8	0x9	4B	4G	4C	4D
9	0xa	Diode	k	n	u
10	0xb	Beep	M	%	m
11	0xc	Hold	Rel	Ohms	Farads
12	0xd	Low battery	Hz	V	A
13	0xe	User Def. 1	User Def. 2	Celsius	Fahrenheit
14	0xf	Max	Min-Max	Min	Auto Power-Off

Note that the segment lettering used here follows the standardised 7-digit notation:

	A
F		B
	G
E		C
	D

Resources

• Datasheet (Chinese)
• Datasheet translation courtesy of Kerry D. Wong who hacked his ennoLogic eM860T True RMS Multimeter to enable RS232.
• Peaktech 3415 software package, contains a file named "PeakTech device communication protocols 2015-07-20.pdf" documenting the protocol corresponding to this chip (among other protocols used by PeakTech).

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.

Example Packet

17 27 3D 4F 5D 67 7D 87 9D A0 B0 C0 D4 E0 -> reads: DC 0.000V

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 Voltcraft M-3650CR use a slightly different protocol:

Packets

Range	00	01	02	03	04	05	06	07	08	09	10	11	12	13	Remarks
All									Sp					\r
D/Beep						O	L			m	V				No contact
				1	2	3	4			m	V				Contact/forward voltage
200 Ω						O	.	L			O	h	m		No contact/overload (invalid value)
				1	2	3	.	4			O	h	m		Value
2 kΩ					.		O	L		k	O	h	m		No contact/overload (invalid value)
				1	.	2	3	4		k	O	h	m
20 kΩ						.	O	L		k	O	h	m		No contact/overload (invalid value)
				1	2	.	3	4		k	O	h	m
200 kΩ						O	.	L		k	O	h	m		No contact/overload (invalid value)
				1	2	3	.	4		k	O	h	m
2 MΩ					.		O	L		M	O	h	m		No contact/overload (invalid value)
				1	.	2	3	4		M	O	h	m
20 MΩ						.	O	L		M	O	h	m		No contact/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 (guessed...)
	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

WENS98A / Voltcraft GDM 704

Protocol

The WENS98A supports two transmission modes, data mode (also "basic mode"), and screenshot mode (also "bit map mode"). Only data mode will be described below.

In data mode, the GDM 704 continuously sends 26 Byte data packets of the following form:

b M . S VVVVV UUUU .. s vvvvv uuuu e

Segment	Position	Length	Description	Example
b	0	1	beginning of packet (constant 0x02)	<0x02>	Packets can be detected in the data stream by looking for 24 "meaningful" bytes enclosed in an <0x02> / <0x03> pair.
M	1	1	mode indicator	A	single letter, A..R
.	2	1	ignore
S	3	1	main value sign		<space> when positive, '-' when negative
VVVVV	4	5	main value	0.025	four digits plus (floating) decimal point, zero-padded
UUUU	9	4	main value unit	Vac	left-aligned
..	13	2	ignore
s	15	1	secondary value sign		<space> when positive, '-' when negative
vvvvv	16	5	secondary value	50	up to five digits, or four digits plus (floating) decimal point, zero-padded or right-alined
uuuu	21	4	secondary value unit	Hz
e	25	1	end of packet (constant 0x03)	<0x03>

The mode indicator can take the following values:

Mode Indicator	Mode Name	Primary Value	Secondary Value	Example
A	Voltage AC	Voltage	Frequency	AB 0.025Vac  B    50 Hz
B	Voltage DC	Voltage	Frequency	BB 0.020Vdc  B     0 Hz
C	Voltage AC (mV / high input impedance)	Voltage	Frequency	CB 000.0mVac B     0 Hz
D	Voltage DC (mV / high input impedance)	Voltage	Frequency	DB 000.0mVdc B     0 Hz
E	Resistance	Resistance	(Frequency)	EB 14.02MOHM B     0 Hz	Frequency is somewhat meaningless and, in my experience, always zero
G	Diode Test	Voltage	(Voltage)	GB 4.000Vdc  B 4.000Vdc	both voltages are identical
H	Capacitance	Capacitance	(Frequency)	HB 0.000nF   B     0 Hz	Frequency is somewhat meaningless and, in my experience, always zero
I	Current AC (mA/µA)	Current	Frequency	IB 003.4mAac B     0 Hz
J	Current DC (mA/µA)	Current	Frequency	JB 000.8mAdc B     0 Hz
K	Current AC (20A)	Current	Frequency	KB 00.10Aac  B     0 Hz
L	Current DC (20A)	Current	Frequency	LB 00.00Adc  B     0 Hz
M	Logic	Voltage?	(Frequency)	MB 04.45     B     0 Hz	Voltage is given without units; Frequency is meaningless
O	Temperature	Temperature (°C)	Temperature (°F)	OB 000.0@C   B  0032@F	with external adapter
P	relative Humidity	rel. Humidity	(Voltage)	PB-000.0%RH  B  5.00Vdc	with external adapter; Voltage is meaningless
Q	Pressure	Pressure (psi)	Pressure (kPA)	QB-000.0psi  B  0000kPa	with external adapter
R	Current (High Current)	Current	Frequency	RB 023.3Aac  B     0 Hz	with external adapter

Modes F, N, S, T are described in the documentation, but not produced by my GDM 704. Instead, the multimeter does not transmit anything in those modes.

Mode Z indicates screenshot mode.

The same protocol is probably also used by the GDM 703 and the GDM 705.

Resources

• GDM 704 User Manual (German)
• GDM 704 Calibration Procedure / Circuit Diagram / Parts List
• GDM 704 Interface Description
• Earlier implementation (as part of the QtDMM project)
• GDM 704 teardown