I am looking into designing a data converter that must provide data from a Scale (usually referred to as an indicator) to a very old PLC that only accepts data in BCD.

The current device that does this is failing and is no longer available (for quite some time now).

BCD isn't the problem, but rather the current device outputs the BCD in TTL to a 50 pin connector. The 50 pins include 6 digits worth of BCD digits along with a Gate signal for each digit and three inputs to the BCD source. (see inputs on pins 37, 39, and 50).

Brief background:

Schematics are of course not available of the current device while those for PLC inputs can be found, but only 'examples'.

Question: Does anyone have experience with a PLC "and" BCD inputs such that you can tell me that the BCD data is "loaded" when one of the inputs to the source of the BCD data (pins 37, 39, or 50) is asserted?

Elaborating on the question: The BCD data is presented in a parallel fashion via the 50 pin connector. I do not have physical access to the current (failing) hardware. (it is 450 miles away. If I did I wouldn't be asking this question.)

In order for me to provide the data to the PLC per the 50 pin data cable I need to know if (again when dealing with BCD data) the PLC asserts one or more of the signals I mentioned (pins 37, 39, or 50) to "latch in" the data. I would imagine that (or something) is used to sync when the PLC "reads" the data in.

This is going to be an "old timers" type of question, this system was installed in 1981.


  • \$\begingroup\$ I've read your question three times and am still confused. Can you rephrase the question at the end? \$\endgroup\$
    – Transistor
    May 8 '19 at 17:04
  • \$\begingroup\$ To summarise: the source of the data is the scale and the old PLC has to read it. You need to program a new man-in-the-middle converter to satisfy the PLC. You suspect that the PLC will periodically issue a "hold" command to freeze the converter while the PLC reads in the BCD data. Is that correct? What do you think pins 37, 39 and 50 have to do with it? I would have thought the PLC would pull 49 low to hold the reading. \$\endgroup\$
    – Transistor
    May 8 '19 at 18:15
  • \$\begingroup\$ That may very well be the way it works, I wondered if anyone "knew" how it worked. BCD was common long ago. \$\endgroup\$
    – Wendall
    May 8 '19 at 18:53
  • \$\begingroup\$ Hi Wendall, A truck connecting rod balancing machine, built in 1990, had a fixture with two weighing scales to measure the big and small end weight. After the part was loaded on to the fixture, and the readings had stabilized, a push button would be operated to register the parallel BCD outputs of the scales via PLC digital inputs. The part would then be loaded on to the machine, to remove the excess material at either end through a series of calculated milling cuts. It was possible to successfully balance the part after three successive cycles of weighing and machining. \$\endgroup\$
    – vu2nan
    Dec 22 '21 at 6:20

I would have to guess you are dealing with binary input to bcd output. A DM74185A per bcd set with a MM74C902 if you need buffering etc.

Its very hard to figure out what you needed. But a binary shift register is what seems to fit your application. Of course, you can do it with 16 jk flip-flops and 4 quad nor gates instead of four 74185 ic chips.

I would guess that you would do the 10's compliment bcd circuit with the 74185.

  • \$\begingroup\$ The mid 1990's PLC (in this case) is made by Allen-Bradley and is used to control an assembly line. The scale that outputs the data has not been made for 15 years. The specifics I am looking for is the "protocol" of how and when the parallel data is clocked into the PLC. Currently I will use a different serial output from the scale and convert the data with an Arduino and a few latches for outputs. The conversion is not a problem, but rather the sequence of events that the PLC (again, very, very old) uses to "read" the data. \$\endgroup\$
    – Wendall
    May 9 '19 at 12:35
  • \$\begingroup\$ What series the AB PLC? 1771/plc5 is software, previously, it was ttl gates with embedded firmware on a 80c51 cpu \$\endgroup\$
    – drtechno
    May 11 '19 at 11:37
  • \$\begingroup\$ I never did learn the exact model/series of PLC. A (new, never used) 17 year old scale with a BCD output module was located, purchased and is currently being used. So no engineering is necessary. Yet! \$\endgroup\$
    – Wendall
    May 20 '19 at 14:38

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