Johnny wrote:
> Minor nitpick: that isn't flow control signals, it's modem control
> signals. People have only perverted them into flow control...


Well, modem control *was* flow control, but only in one direction.
The DTE (data terminal equipment) is supposed to assert RTS when you have
data you're ready to send, and the DCE (data communication equipment,
i.e., modem) asserts CTS when it's ready for the DTE to send it. But
there isn't a corresponding handshake for receive data. Some people
used DTR/DSR for the receive flow control, but that wasn't what they
were for.

Modern (as you call it, "perverted") usage is for the DTE to assert RTS
to indicate ready to receive, and the DEC assumes that the DTE is always
requesting to send, so it generates CTS without regard to RTS. Thus
there is flow control in both directions, using only a single line in
each direction. This was not part of the original RS-232 standard, but
reportedly is part of the EIA/TIA-232-E and EIA/TIA-232-F standards. Thus
it is no longer "perverted", but in fact standard. :-)

There is even a more recent EIA/TIA standard that renames the RTS signal
to a new name more descriptive of this new usage. I can't find the
standard at the moment, and I'm not certain what the new name is, though
it might be "RFD" (Ready For Data).

Eric
----------
To unsubscribe (or subscribe) from (to) this list, send a message to
info-pdp11-request@village.org, with the first line of the message
body being "unsubscribe" or "subscribe", respectively (without the quotes).