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I have several Tenma model 72-6870 DMMs in our physics lab that use a 2 wire data transfer set up. Does anyone have any information that could leed me to understanding this type of data protocol? Also, is there any information on the 5 wire Tenma model 72-7755 DMM RS232 Protocol that anyone could share with me? This meter interfaces with a computer via the RS232 connector on a PC. Software is supplied by Tenma to log readings from the DMM. My problem is "how to interface the latter 72-7755 DMM to our 4 1/2 digital display units that have been working with former 72-6870 DMM"

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    \$\begingroup\$ I'm guessing that the manufacturer would have this in some manual somewhere. \$\endgroup\$
    – Nick Alexeev
    Mar 1, 2016 at 16:37
  • \$\begingroup\$ Can't be too complicated to attach a scope and correlate the information with that displayed on the pc \$\endgroup\$
    – PlasmaHH
    Mar 1, 2016 at 16:47
  • \$\begingroup\$ @NickAlexeev The manufacturer might, but with TENMA stuff, the manufacturer isn't TENMA themselves (or at least not any more). \$\endgroup\$
    – Asmyldof
    Mar 1, 2016 at 17:32
  • \$\begingroup\$ mmm, TENMA appears to be a house brand of the Premier Farnell group (their address is the same as MCM) selling badge-engineered gear. \$\endgroup\$
    – Peter Green
    Jun 18, 2016 at 0:33

1 Answer 1

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A quick Google search led me to a forum discussion on the topic of the 72-6870 protocol. Verbatim copy/paste follows.

The Tenma periodically sends out a packet of 14 bytes of data without being prompted. The parameters are 2400 bps, 8 bits, no parity, one stop bit. The upper nybble of each byte contains the sequence number of the byte, starting with 1. That is, the first byte (after a pause) is 0001XXXX, the next is 0010XXXX, the third is 0011XXXX, etc. The lower nybble is the data that drives the LCD display. In the following, bit 7 is the most significant bit, bit 0 is the least significant bit.

BYTE 1
BIT
7 - 0
6 - 0
5 - 0
4 - 1
3 - AC
2 - DC
1 - AUTO (RANGE)
0 - 1


BYTE 2 (LEFTMOST LCD DISPLAY DIGIT)
BIT
 7 - 0
 6 - 0
 5 - 1
 4 - 0
 3 - SIGN
 2 - LOWER LEFT VERTICAL SEGMENT
 1 - UPPER LEFT VERTICAL SEGMENT
 0 - TOP HORIZONTAL SEGMENT


BYTE 3 (LEFTMOST LCD DISPLAY DIGIT)
BIT
 7 - 0
 6 - 0
 5 - 1
 4 - 1
 3 - BOTTOM HORIZONTAL SEGMENT
 2 - LOWER RIGHT VERTICAL SEGMENT
 1 - CENTER HORIZONTAL SEGMENT
 0 - UPPER RIGHT VERTICAL SEGMENT


BYTE 4 (SECOND LCD DISPLAY DIGIT FROM LEFT)
BIT
 7 - 0
 6 - 1
 5 - 0
 4 - 0
 3 - DECIMAL POINT
 2 - LOWER LEFT VERTICAL SEGMENT
 1 - UPPER LEFT VERTICAL SEGMENT
 0 - TOP HORIZONTAL SEGMENT


BYTE 5 (SECOND LCD DISPLAY DIGIT FROM LEFT)
BIT
 7 - 0
 6 - 1
 5 - 0
 4 - 1
 3 - BOTTOM HORIZONTAL SEGMENT
 2 - LOWER RIGHT VERTICAL SEGMENT
 1 - CENTER HORIZONTAL SEGMENT
 0 - UPPER RIGHT VERTICAL SEGMENT


BYTE 6 (THIRD DISPLAY DIGIT FROM LEFT)
BIT
 7 - 0
 6 - 1
 5 - 1
 4 - 0
 3 - DECIMAL POINT
 2 - LOWER LEFT VERTICAL SEGMENT
 1 - UPPER LEFT VERTICAL SEGMENT
 0 - TOP HORIZONTAL SEGMENT


BYTE 7  (THIRD LCD DISPLAY DIGIT FROM LEFT)
BIT
 7 - 0
 6 - 1
 5 - 1
 4 - 1
 3 - BOTTOM HORIZONTAL SEGMENT
 2 - LOWER RIGHT VERTICAL SEGMENT
 1 - CENTER HORIZONTAL SEGMENT
 0 - UPPER RIGHT VERTICAL SEGMENT

BYTE 8 (RIGHTMOST LCD DISPLAY DIGIT)
BIT
 7 - 1
 6 - 0
 5 - 0
 4 - 0
 3 - DECIMAL POINT
 2 - LOWER LEFT VERTICAL SEGMENT
 1 - UPPER LEFT VERTICAL SEGMENT
 0 - TOP HORIZONTAL SEGMENT


BYTE 9  (RIGHTMOST LCD DISPLAY DIGIT)
BIT
 7 - 1
 6 - 0
 5 - 0
 4 - 1
 3 - BOTTOM HORIZONTAL SEGMENT
 2 - LOWER RIGHT VERTICAL SEGMENT
 1 - CENTER HORIZONTAL SEGMENT
 0 - UPPER RIGHT VERTICAL SEGMENT


BYTE 10
BIT
 7 - 1
 6 - 0
 5 - 1
 4 - 0
 3 - u (MICRO) INDICATOR
 2 - n (NANO) INDICATOR
 1 - k (KILO) INDICATOR
 0 - DIODE SYMBOL INDICATOR


BYTE 11
BIT
 7 - 1
 6 - 0
 5 - 1
 4 - 1
 3 - m (MILLI) INDICATOR
 2 - unused
 1 - M (MEGA) INDICATOR
 0 - TONE ANNUNCIATOR SYMBOL


BYTE 12
BIT
 7 - 1
 6 - 1
 5 - 0
 4 - 0
 3 - F (FARAD) INDICATOR
 2 - OMEGA INDICATOR
 1 - unused
 0 - unused


BYTE 13
BIT
 7 - 1
 6 - 1
 5 - 0
 4 - 1
 3 - A (AMPERE) INDICATOR
 2 - V (VOLT) INDICATOR
 1 - Hz INDICATOR
 0 - unused


BYTE 14
BIT
 7 - 1
 6 - 1
 5 - 1
 4 - 0
 3 - unused
 2 - DEGREES C INDICATOR
 1 - unused
 0 - 1
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    \$\begingroup\$ So connecting the incredibly idiotic LCD to the other proper-RS232 one will take a string-parsing MCU that spits out the above utter pile of nonsense. LOL at buying TENMA. That's what happens when a company buys random crap from random suppliers and re-badges it as their name. \$\endgroup\$
    – Asmyldof
    Mar 1, 2016 at 17:31
  • \$\begingroup\$ Yeah. Scary protocol. What do people have against SCPI anyway? Yeesh. \$\endgroup\$
    – Adam Lawrence
    Mar 1, 2016 at 19:25
  • \$\begingroup\$ I wouldn't care if it's SCPI or any other ASCII type interface, but the "LCD-Segment-Interface"(tm) is just absolute nonsense. Who wants that? Even a Binary protocol that just sends a packet of 4 BCD numbers preambled by a start byte would be fine in the grand scheme of things, you'd at least be able to make some programmatic sense of it. \$\endgroup\$
    – Asmyldof
    Mar 2, 2016 at 9:51
  • \$\begingroup\$ Adam, You da MAN! This is exactly on point and your help has moved me off Top Dead Center to borrow a mechanical engineering phrase. Many thanks, sir! Randy \$\endgroup\$
    – Randy Penn
    Mar 2, 2016 at 18:13

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