The RS-232 standard allows a secondary channel on a 25-pin connector. Are there any examples where this secondary channel is actually used, in computers resp. devices connected via the serial port? Googling for it mainly turns up "some modems, for administrative purposes, on a slower connection", but I couldn't find specifics, or examples for computers providing it.

3 Answers 3


You may not be seeing specific platform examples because the use of the secondary channel (like all Standard serial comms) is essentially platform agnostic. It would be found on systems where comms with the remote end, usually a modem-type device, needed to have the baud rate reset or settings adjusted because of parity errors detected at either end (among others).

So, you'd find this used in places where they had a pool of modems, or a critical modem connection to another network. It would also be used on specialty equipment related to things like telephony.

Typically, the modem-type (DCE) equipment would implement this in firmware or hardware via a UART of some sort, and the control end (DTE) could be anything that understood serials comms.

The specs for these are rather obscure and just don't show up on casual web searches.

I thought HP used this for their proprietary serial networking protocol, but I can't find any confirmation of this. It might have just been plain RS-422/485. (Though the secondary line could have been used there, too. I just can't find evidence that it was for any regular HP equipment from the late 80s.)


In addition to things the other answers have mentioned, in the early days of data communication the main channel was sometimes half duplex because the electronics weren't up to more, but there was a secondary channel at a much slower data rate and this line in the RS232 cable was used to control it. BTW, the kind of numbers I'm talking about are a high speed channel at 600 bps half duplex and a secondary channel with rates as low as 1 bps, in that case primarily used to send the one bit message "stop sending, I urgently need to tell you something".

  • 1
    Do you know any details about what kind of hardware was actually used (if you know the actual data rates)?
    – dirkt
    Jun 30, 2016 at 4:45
  • @dirkt: No, I never used that stuff, but I heard about it a lot when I was first learning Data Comm. If I new where my copy of TADC wandered to, I'd check there. I think John mentions some of this, and he certainly saw it in his years of designing data comm. gear for DEC.
    – MAP
    Jul 2, 2016 at 6:05
  • @dirkt these sound like acoustic couplers for analog phone line modems. Jul 4, 2016 at 16:18
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    @RBarryYoung: Acoustic couplers were _never _ sophisticated enough to make any use of the secondary channel. The constraints posed by actually making a sound and then having the phone convert it meant that you couldn't do any kind of sophisticated signalling. In fact, they were sometimes problematical if you tried to push them to 600 bps.
    – MAP
    Jul 4, 2016 at 20:56

The only information I found so far is about Telebit modems where the secondary channel was used to query modem status while the primary channel was busy with data.

This link reports an email from Telebit staff on configuration guides and mentions several supported machines - although there is no mention of secondary channel it's likely that some of those machines could have had it provided.

  • I googled a description of a Sun setup, and they only used a single channel (ttya). So, more details needed. :-)
    – dirkt
    Jun 28, 2016 at 13:56
  • I don't think this is the same thing at all. I had a trailblazer (in fact it might even be in storage) and the wikipedia article reads more like it's talking about the dynamic nature of the upstream / downstream bandwidth usage (similar to the 1200/75 & fax modem standards). The trailblazer could fake x/ymodem, uucp and maybe kermit handshakes so there was no waiting for acknowledges from the other end. I used it originally (1987 at a guess) to transfer files from the US office of Borland back to the UK.
    – PeterI
    Jun 30, 2016 at 14:27
  • @Peterl: Is this the model that had the two serial interfaces (which seemed to be used mostly in data centers), or an end-user model? If it's in storage, could you have a look?
    – dirkt
    Jul 2, 2016 at 5:43
  • @dirkt this sounds more like 60s and 70s stuff. By the 80s hardly anyone was using all 25 pins anymore. Jul 4, 2016 at 16:20

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