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I'm sorry if this is the wrong use for stack exchange. I'm sure I will be notified by users if it is.

I have inherited a (female) DSUB9 to (female) DSUB9 cable from a very old project. The pinning perplexes me. What pinning is this? I mean, what is the name of this arrangement? I want to order at least one more. I could just deal with it and make an adapter for some female-to-male cable I have, but I want to understand.

EDIT: It's used to programmed a Cypress Semiconductor MCU CY90F543GSPFR-GE1

The cable is correct by the way, both pin 6 and 8 on one side go to pin 4 on the other. I figured it must be null-modem, but it does not match, as far as I can tell.

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DrOnline
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1 Answers1

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It's close to a plain old null modem cable.

DB9 female according to RS-232:

  1. DCD (data carrier detect)
  2. Tx (transmit, pair with Rx)
  3. Rx (receive, pair with Tx)
  4. DTR (data terminal ready, pair with DSR)
  5. GND (signal ground)
  6. DSR (data set ready, pair with DTR)
  7. CTS (clear to send, pair with RTS)
  8. RTS (ready to send, pair with CTS)
  9. RI (ring indicator)

Everything in your pin list matches this except on a proper null modem, you are supposed to connect CTS with RTS rather than with DCD. So you appear to have some specialized, non-standard version. It's somewhat common in non-standard pinouts to give supply voltage on one pin, such as 1.

Edit: in case this was used for MCU programming over RS-232 bootloader, then it's quite likely that some of these pins are used for reset, reference voltages, mode select or similar. Older flash parts often required a high programming voltage > TTL levels at some pin.

Lundin
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  • There are approximately as many different crossover cable wirings as there are different pairs of equipment needed to be connected together, so there is no kind of "standard" nullmodem cable. I consider all of them "special", but yes there are a few very typical wirings. Even simply connecting two standard PCs together is not trivial, as the software makes it special. Software running on it can make assumptions about the other device which the cable must support with correctly simulating the behaviour. – Justme Apr 20 '21 at 10:30
  • @Justme No, this is properly standardized in one of the ancient X.25 standards. I can't be bothered to look it up, if I even have access to those still. – Lundin Apr 20 '21 at 10:33
  • @Lundin Thanks a lot. I feel like an idiot. Yes this cable does match some old cable I found, when I double checked. Someone had labelled this found cable "crossed". The person must have meant null-modem. Now I have two cables, at least doubling the size of my production bottle neck. Thanks. I've marked your post as solution, of course. – DrOnline Apr 20 '21 at 10:36
  • Updated after edit by OP. Very likely some old evil prog interface. One of those truly horrible things that existed before JTAG and single wire interfaces. – Lundin Apr 20 '21 at 10:36
  • @Lundin The board design is old, but not THAT old.. 2005? – DrOnline Apr 20 '21 at 10:37
  • @DrOnline The evil interfaces were still around then. I used to struggle with Motorola 68HC08 around then and it was a fully modern part with flash etc, except for the prog interface which was crafted by bumbling dinosaurs. It wasn't until around 2008 Motorola released a new family where the interface was replaced with a proprietary single-wire one. I'd assume Cypress parts roughly follow the same time line. – Lundin Apr 20 '21 at 10:40