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   argv minus one (argv_minus_one@mastodon.sdf.org)'s status on Wednesday, 26-Mar-2025 08:58:28 JST argv minus one

   The #Hayes #Smartmodem had a separate data mode (in which bytes are sent/received over the phone line) and command mode (in which the computer sent commands to the modem itself).

   Switching from command to data mode was simple: send the command `ATO`.

   The reverse was more complicated. The modem would switch from data mode to command mode whenever the computer tried to send the special character sequence `+++`, with a delay before and after the sequence.

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   #RS232 #modem #retrocomputing

   • Embed this notice
     tom jennings (tomjennings@tldr.nettime.org)'s status on Wednesday, 26-Mar-2025 08:58:25 JST tom jennings

     in reply to

     @argv_minus_one

     That would fail: binary file transfers toss all 256 combos at the port. Compression assured the spread.

     In practice, delay +++ delay worked perfectly. I don't think it happened even once to me or anyone I talked to (very large number).

     Also the worst that could happen was disconnect; the host could talk to the modem but from the line side, nada.

     Modem control and recovery was a deepish art in the FidoNet days. They weren't that determinate when things Went Wrong. And there were like 20 or 50 different modem types in use. Some were just awful. Some were wildly complex. Some could not be used as a BBS incoming.

     Getting serial ports with consistent control lines as per anything like rs232 was awful. The damn apple machine had zero handshake lines. Things were wildly inconsistent then.

     Haelwenn /элвэн/ :triskell: likes this.
   • Embed this notice
     argv minus one (argv_minus_one@mastodon.sdf.org)'s status on Wednesday, 26-Mar-2025 08:58:26 JST argv minus one

     in reply to

     'Course, I'm about 45 years too late to be having this thought, but… 🤷♂️

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     argv minus one (argv_minus_one@mastodon.sdf.org)'s status on Wednesday, 26-Mar-2025 08:58:26 JST argv minus one

     in reply to

     Another possibility comes to mind: use a single-byte escape code instead (ASCII's Data Link Escape code 16 seems appropriate), and add a modem command that sends that literal byte and goes back to data mode.

     So, for example, whenever you need to transmit `␐`, you'd instead transmit `␐ATY␍` (for a hypothetical command `ATY` that sends a literal ␐ and switches back to online mode) and then continue transmitting as normal.

   • Embed this notice
     argv minus one (argv_minus_one@mastodon.sdf.org)'s status on Wednesday, 26-Mar-2025 08:58:27 JST argv minus one

     in reply to

     Unless I'm mistaken, DTR is kept active at all times while the computer is using the modem. Its only function is that the modem is to hang up if DTR becomes inactive for 2 seconds.

     So, if DTR becomes inactive for only 0.5 seconds and then becomes active again, what happens then? If the answer is nothing, then that could be interpreted as a signal to switch the modem to command mode.

     Nobody can trick you into toggling DTR, so that should be fine, no?

     3/end

   • Embed this notice
     argv minus one (argv_minus_one@mastodon.sdf.org)'s status on Wednesday, 26-Mar-2025 08:58:28 JST argv minus one

     in reply to

     This was patent encumbered and also a DoS vulnerability: tricking anyone into transmitting that sequence would knock them offline!

     There must have been a better solution than this.

     One possibility comes to mind: switch modes when the computer briefly toggles the DTR signal. https://en.wikipedia.org/wiki/Data_Terminal_Ready

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