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Multimeters (DMM) and their chips, with computer communication
            Control chips
            Serial output types
            Thoughts on 7106
            Identification of Epoxy Blob Chips
            Safety devices
      Schematics
      List of chips and the meters using them
            With serial output (native RS232, USB, or hackable)
                  19C6RVT/M430F448 
                  BTC AD-85-4
                  CS7721CN
                  DTM0660L (DreamTech)
                  DM1106EN
                  EF9922-DMM4
                  ES51862
                  ES51922
                  ES51932
                  ES51960
                  ES51962
                  ES51966F
                  ES51966P
                  ES51968
                  ES51978
                  ES51986A
                  ES51997
                  FS9711A
                  FS9721-LP3 (Fortune Semiconductor)
                  FS98o24
                  FS9922-DMM3
                  FS9922-DMM4
                  GC7721AQ-P3
                  HCPD608 (Hung Change)
                  HY3131 (Hycon)
                  Mastech M343-01
                  Metex 89CR
                  Metex KS57C2016
                  SH7108
                  UM7108
                  UP01+FS970X
                  WENS98A
            Unknown serial-out presence
            Without serial output
                  71xx series
                  mysterious epoxy blob, usually without serial output, often 7106
                  7106
                  7129
                  CS7721
                  ES5106E
                  FS9711-LP1
                  FS9721-LP1
                  FS9952
                  KAD0501
                  KAD7001CQ:
                  NJU9207
                  SMC62MIF
            Unknown serial-out presence
            Without serial output
                  mysterious epoxy blob, usually without serial output, often 7106
                  7106
                  7108
                  7124
                  7126
                  7129
                  7136
                  7149
                  CS7721
                  ES5106E
                  FS9711-LP1
                  FS9721-LP1
                  M5230
                  M6266
                  M6306
            By brand
                  UT
                  Mastech
      Table of chips and pins
      Protective circuit notes
            known-bad meters
            fuses
            voltage clamping with transistors
            voltage clamping with forward-biased diodes
            LCR meter protection
      Useful links

Multimeters (DMM) and their chips, with computer communication

There are many brands and types of multimeters. There are considerably fewer types of chips the meters are built around. Some meters, usually the more expensive ones, have built-in serial communication, usually optically separated UART. The UART can be converted to RS232, or, in more modern systems, to USB by several possible means - some meters use USB-serial converters and appear like a serial port, others use USB-HID chips. Yet others use bluetooth-serial interface modules.

Control chips

The multimeters are based on three kinds of control chips:

Serial output types

The RS232 meters usually use an optocoupler, or a discrete LED-phototransistor pair, for communication. The interface has two layers - the low-voltage UART, and the low or higher level protocol interface to the host computer.

The hackable meters are built around a chip that offers serial output, but the pin is not connected and function not enabled. It is possible to attach an optocoupler (maybe directly, most often transistor-driven). The serial output is rarely always-on, usually has to be enabled by momentary pulse or permanent connection (to ground, V-, or other power rail, see datasheet) on another pin.

As the base interface with the meter is usually a standard UART, even meters that come with a wrong type of interface can be usually converted to something else. Just find where the serial signal is (usually at the connector, or at an optocoupler), and attach the right one. (E.g. replace a dumb nonconfigurable USB-serial chip with a smarter one with an EEPROM that can be reprogrammed to report itself as the multimeter, or a HID one can be replaced with a serial one, to avoid the need for a driver. Or a RS485 transceiver can be added for a long-haul wired data transfer. Or a UART-wifi chip can be added for wireless logging. Or a Bluetooth serial interface, for connection to a smartphone; potentially even the internal multimeter's battery could be used here, as the wireless interface provides the isolation.)

Thoughts on 7106

The cheapest of cheapest meters use a simple ADC integrated with a LCD driver, usually a 7106 or similar chip. These are virtually nonhackable, spare connecting directly to the LCD output pins. There are typically 23 connections to the display, which makes this solution rather intensive in regards to the pins needed, and the AC-driving nature of the LCD (signal to segments either in-phase (off) or out-of-phase (on) with the backplane - sustained DC bias would damage the liquid crystals) adds to the unpleasantness. For 48kHz clock (3 readings per second) the LCD drive frequency is typically 60 Hz (clock divided by 800).

A direct-segment readout can be done either with a microcontroller that has enough spare pins, a lightweight FPGA that does the connection and decoding, a parallel-to-serial shift register to which the display data are latched and then clocked out serially, or a multiplexer for reading the segments one by one (together with the backplane signal, or even XORing it in hardware to get the direct on/off state of the segment).

When not sampling all the lines at once, it is important to take care of the changes in the backplane level. As the frequency is fairly low, the microcontroller can either watch for a change and then do all the sampling, or check before and after and if there is a mismatch the backplane flipped polarity during data acquisition and the data have to be discarded.

Or the chip can be entirely bypassed, and a separate ADC (e.g. a microcontroller input) connected to the 7106's analog input. The readings then can vary a little between the ADC and the 7106's display but the circuitry is way simpler.

Yet another approach is optical, using a camera and an image analysis software (eg. OpenCV can be helpful here).

Identification of Epoxy Blob Chips

The cheapo ones also tend to use the Unidentifiable Epoxy Blob chip packaging. This is particularly annoying. Sometimes the chip can be identified indirectly, as the blob sits on the board in a rectangle of solder pads where a more sanely packaged chip can be alternatively soldered on.

If the rectangle-o'-pads is available, counting the chip pins is easy. Start there and pick only the datasheets that match pin count.

The easiest pads are the ones to crystal or resonator. The xtal is nearby and easy to identify. If it is present, start there.

Other easiest pads/tracks to trace are those to the display pads. Identify these, compare their number and layout (and display multiplexing scheme) to the datasheets of the candidate chips. Repeat with power pins and any other signals you can trace, until you have no more datasheet left and the chip remains unidentified. (In rare cases you get a positive identification.)

If there are not too many pins, start with the crappy 7106 chip (40-pin), as it and its clones are the most common in these scenarios.

Safety devices

An important part of a meter, or a data acquisition unit in general, is the capability to withstand the unexpected. Dave Jones of EEVblog does an excellent job describing these.

Some known-bad meters are listed here.

In short, the important parts are:


Schematics

Various schematics culled from The Internets.


List of chips and the meters using them

With serial output (native RS232, USB, or hackable)

19C6RVT/M430F448 

(custom microcontroller)

BTC AD-85-4

CS7721CN

see FS9721-LP3

DTM0660L (DreamTech)

brochure, datasheet, from http://www.kerrywong.com/2016/03/19/hacking-dtm0660l-based-multimeters/
LQFP-64, serial out, TX on pin 20, xtal on 15/16, SPI on pins 18(SCK)/19(SDO)/22(SDI),23(SCE) 16x16 square pins, can be an epoxy blob needs EEPROM bit switch to enable UART output: arduino flashing sketch
clone of Hycon HY12P65?)

DM1106EN

(maybe cloned/rebranded HyconTek HY12P66?)

EF9922-DMM4

ES51862

ES51922

datasheet QFP-128, serial-out, SDO at pin 123, RS232enable at pin 111

ES51932

ES51960

ES51962

datasheet (serial-out, SDO at pin 94, RS232enable at pin 45)

ES51966F

datasheet QFP-64, separate ADC, needs additional microcontroller for display driving and output, talks via STATUS/SCLK pins, protocol described in the datasheet

ES51966P

datasheet QFP-64, analog front-end, needs additional microcontroller for display driving and output, talks via STATUS/SCLK pins, protocol described in the datasheet

ES51968

ES51978

datasheet QFP-100, serial-out, SDO at pin 94, RS232enable at pin 45

ES51986A

ES51997

analog front-end

FS9711A

QFP-100, TxD on pin 64, TXen on 84

FS9721-LP3 (Fortune Semiconductor)

datasheet QFP-100, serial-out, Tx on pin 64, TXen on 84, 2400bit

FS98o24

OTP microcontroller, unspecified functionality

FS9922-DMM3

QFP-100, datasheet Tx on pin 92, TX-enable on pin 94 (beeper output has delay?)

FS9922-DMM4

QFP-100, datasheet v11, FS9922-DMM4-DS-14_EN.pdf TX on pin 92, TX-enable on pin 94
(beeper output has delay?)

(note: VC99 has slow readout speed. Modification as described here: replacing three SMD ceramic capacitors with film 0.01uF / 100 V. By designation on the board (you need to remove the display - 4 self-tapping screw) - C16 (between 5 and 6 pins FS9922), C17 (between 7 and 8 pins FS9922) and C7 (between 17 pins FS9922 and ground). Capacitor C7 affects the measurement of resistances.)

GC7721AQ-P3

see FS9721-LP3

HCPD608 (Hung Change)

Tx/Rx, 9600 baud, 7n1

HY3131 (Hycon)

datasheet DMM analog front-end, no display, SPI interface

Mastech M343-01

Metex 89CR

Metex meters tend to use the same protocol: http://sigrok.org/wiki/Multimeter_ICs#Metex_14-byte_ASCII

Metex KS57C2016

(Samsung KS57C2016 4-bit microcontroller with custom Metex firmware)

SH7108

datasheet serial output, non-RS232, needs external clock signal on CLK (36), output from pin 37, EOC (pin 39) goes to H at end of conversion when fresh data in buffer; 16 bits of data in buffer, with digits in BCD code, two bits for decimal point, one for polarity, no indication of modes nor anything else

UM7108

see SH7108

UP01+FS970X

likely some custom-programmed microcontroller

WENS98A

protocol

Unknown serial-out presence

Without serial output

71xx series

https://hackaday.com/2017/01/31/get-to-know-3%C2%BD-digit-adcs-with-the-icl71xx/

mysterious epoxy blob, usually without serial output, often 7106

7106

7129

(7106-like, with one more digit)

CS7721

see FS9721

ES5106E

(7106-like)

FS9711-LP1

Essentially identical to FS9721-LP1.
A variant of FS9721-LP3, with the UART pins unused.

FS9721-LP1

A variant of FS9721-LP3, with the UART pins unused.
datasheet

FS9952

KAD0501

KAD7001CQ:

NJU9207

datasheet

SMC62MIF

Unknown serial-out presence

Without serial output

mysterious epoxy blob, usually without serial output, often 7106

7106

Various schematics here

7108

7124

7126

7129

(7106-like, with one more digit)

7136

7149

CS7721

see FS9721

ES5106E

(7106-like)

FS9711-LP1

Essentially identical to FS9721-LP1.
A variant of FS9721-LP3, with the UART pins unused.

FS9721-LP1

A variant of FS9721-LP3, with the UART pins unused.
datasheet

M5230

M6266

M6306

By brand

UT

Mastech


Table of chips and pins

For identification of potential candidates, where a mystery epoxy blob is hiding in the middle of a pin pattern

chip              package/pins          xtal    txout   txenable   display-seg+com+bias
ES51922           QFP-128   38x26       106,107   123     111         31+4+1
FS9721-LP3        QFP-100   30x20       61,62      64      84         14+4
FS9721-LP1        QFP-100   30x20       61,62      -       -          14+4
FS9922-DMM3       QFP-100   30x20       86,87      92      94         (27+4+1 ? )
FS9922-DMM4       QFP-100   30x20       86,87      92      94         32+4+1
ES51962           QFP-100   30x20       79,80      94      45         26+4+1
ES51978           QFP-100   30x20       79,80      94      45         26+4+1
NJU9207B          QFP-80    24x16       49,50      -       -          10+4+1
ES51966           QFP-64    16x16       50,51      54/55   not-uart   n/a
DSM0660           LQFP-64   16x16       15,16      20      EEPROM     14+4+1
SH7108            QFP-48    12x12       5,6(4=RC)  36/37   not-uart   11+3
7106              QFP-44    11x11       6,7(4=RC)  -       -          23+1
7106              DIP-40     40x2       40,39(38=rc) -     -          23+1

7107 is like 7106 but for driving LED display instead of LCD also ICL7106, CS7106, ICL7107, UM7108 - many many clones 7106 drives LCD directly, 7108 uses multiplexing also something 11x12, rectangular


Protective circuit notes

known-bad meters

fuses

voltage clamping with transistors

voltage clamping with forward-biased diodes

LCR meter protection


Useful links


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