I have a very limited number of outputs available (4 to be exact) on my existing PLC and need to share 5-7 bits of information to another PLC without having the expense off add-on communication cards.

I was thinking of using a binary coded decimal system to turn 3 outputs into 7 separate messages. The problem is that its not just one message at a time, I need to be able to share a combination of ANY 7 of those messages and be able to decode them without fail.

Is there an existing encoding convention that I can use to achieve this?

  • \$\begingroup\$ Add a (maybe less expensive) Digital Output Card? \$\endgroup\$ – Tyler Jul 25 '18 at 18:20
  • \$\begingroup\$ Can you use serial communication or must all of the different messages be simultaneously available? \$\endgroup\$ – Elliot Alderson Jul 25 '18 at 18:33
  • \$\begingroup\$ If you need to send 7 bits of information with only 4 bits, the only solution is to send them sequentially. \$\endgroup\$ – crj11 Jul 25 '18 at 19:07
  • \$\begingroup\$ Note that even in a parallel scheme, you'd probably need to dedicate one bit as a strobe to indicate the validity of the others, or else time-filter the inputs, or use one-hot states, so that you don't get false states on the transition between one code and another. \$\endgroup\$ – Chris Stratton Jul 25 '18 at 21:33
  • \$\begingroup\$ @ChrisStratton A strobe bit is definitely the answer. I strobe in binary messages no problem. \$\endgroup\$ – uMinded Jul 27 '18 at 5:11

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