I'm tapping the ribbon cable interface between a HVAC control board (relays, etc) and display board (membrane panel + led display) to reverse engineer how to change the set point programmatically. I came across these traces using a logic probe / oscilloscope. Small pulses are clearly occuring when keypresses occur, but I can't understand why they occur in reference to what appears to be a 60 hrz signal. My best guess is that somehow the AC line power is getting picked up by the logic probe but I can't see how since I see this pattern regardless if I ground to the chassis or a persistently zero volt pin on the wiring hardness.

Is there a serial communication protocol that looks like this? (Signal appears to be on channel 3 and 4).

Have already googled extensively for specs on a Ranco 7221081 PTAC control board and found nothing hence the need for reverse engineering.

Scope Trace

  • \$\begingroup\$ Looks like channel 3 and 4 are differential pair. And the 60 Hz signal is some kind of common-mode noise (or you have your reference wrong) \$\endgroup\$
    – Eugene Sh.
    Jan 29, 2021 at 17:43
  • 3
    \$\begingroup\$ 'Hz' is the abbreviation for the SI unit 'hertz'. Note capitalisation in both cases. Capitals matter! \$\endgroup\$
    – Transistor
    Jan 29, 2021 at 17:45
  • \$\begingroup\$ Interestingly 3 and 4 were not a differential pair. I had been expecting that too but it turns out they are plain 5V non-inverted TTL/5V serial signals, TX and RX separately, 8N1 4800 baud with an extra 9th signaling bit for address identification. \$\endgroup\$ Jan 30, 2021 at 17:18

1 Answer 1


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It seems to be a faulty signal capture with an induced hum between all signals and the 0V reference pins. Excessive lead length and coupling to finger which radiates E-Fields between the two.

  • \$\begingroup\$ Can you suggest an approach to definitively test which pin is the signal ground? Or do I just systematically try all of them? \$\endgroup\$ Jan 29, 2021 at 20:54
  • \$\begingroup\$ How about an Ohmmeter to most likely ground to pins being tested \$\endgroup\$ Jan 29, 2021 at 21:00
  • \$\begingroup\$ Through some process of elimination managed to figure out the ground and yes, the signal cleaned up. I had been triggering an immediate cold shutdown of the HVAC unit when I had previously attempted to place any leads on this pin, even just an ohmmeter, so I had mentally decided to avoid touching it again. But once I connected the logic analyzer up using that as GND (and ran the laptop only on battery power) I got clean, readable serial traffic. Tnx for the help. \$\endgroup\$ Jan 30, 2021 at 17:15

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