Help needed with 6551-based RS-232 Pak to 3-wire without DCD

Updates:

  • 2018-02-05: Thanks to Dave Chesek, this issue has been solved. He took the wiring diagram below and modified his RS232-to-TTL adapter with a  bit of solder. He put a blob connecting pins 7 (RTS) and 8 (CTS), and then soldered together 1 (DCD) and 6 (DSR) then ran a jumper wire from those two over to 4 (DTR). It worked fine. My efforts to do this with jumper wires must not have been making good connection (perhaps one of my DB9 pin holes is not as deep – TX, RX and GND all worked fine with the jumper wires). I will do a bit more investigation to see what the minimum required. Thanks, Dave!
  • 2018-02-06: See the complete writeup.

The 6551 serial chip will not receive unless DCD is active. Modern adapters do not provide DCD. I am trying to find a way to do this so I can hook my WiFi device (which only has TX, RX and GND) to the RS-232 Pak.

I have been trying to jumper wires to fake the DCD signal but nothing has worked. I do not know if my Pak is defective, since I have nothing else to connect it to for testing,

Many sites suggest a wiring example like this:

Has anyone done this? Nothing modern has DCD so surely there is a simple solution figured out long ago.

2 thoughts on “Help needed with 6551-based RS-232 Pak to 3-wire without DCD

    1. Allen Huffman Post author

      The old Radio Shack RS-232 Pak has a DB25 on it, and I am using a DB25-to-DB9 cable I bought from Amazon last week. Hopefully I am good. The last time I used this Pak was with a modem around 1999. I am expecting something has just gone bad in my 1980s RS-232 interface.

      Reply

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