>> - conventional RTS/CTS
> RTS active = ready to receive
> CTS active = allowed to send
>
>> - alternative RTS/CTS
> RTS active = on before send, off after send
> CTS active = allowed to send
>
>> - RS485
> RTS active = on before send, off after send (RTS enables driver)
> CTS ignored (2 wire mode, no CTS)
>
> So maybe the extra control fields would be:
> CRTSONTX - RTS on before send, off after send
> CTXONCTS - wait for CTS before sending
As someone who's actually used both kinds of modems, here are
the issues:
- For RS-485, you have a half-duplex wire, and the response is triggered
by the serial data. And some of those industrial controllers respond in
less than a character time. You MUST disable the transmitter promptly
when finished sending so you can hear the response.
It's also helpful if the receiver is disabled while the transmitter
is enabled, but that's negotible.
- For Classic half-duplex RTS/CTS, the DTE (computer) must always accept
data, but raises RTS when it wants to send. When it gets CTS, it's
allowed to actually send. There are still single-frequency VHF radio
modems floating around that work this way. When a modem receives
RTS and is not receiving a carrier (CD is deasserted), it enables its
transmitter, and waits a programmed receiver-acquisition delay before
asserting CTS.
Both of these are variants on the same theme, and I'd suggest expressing
them with one additional bit along with the existing CRTSCTS. I'll call
it CRTSHDX (RTS half-duplex). It means "assert RTS when we have data
to send, and deassert it when we don't". When it's not set, RTS is
asserted when we can accept data and deasserted when we can't.
All four combinations are sensible:
CRTSCTS CRTSHDX Handshaking
off off None. (Computer might as well send RTS< but ignores CTS)
on off Full-duplex RTS/CTS
off on RS-485. CTS ignored, RTS enables transmitter.
on on RS-232 half-duplex. RTS is request, CTS is grant.
The upshot is that CRTSCTS controls whether CTS is listened to, and the
new CRTSHDX controls the interpretation of RTS. For a three-wire hookup,
CRTSRCS must be off and CRTSHDX has no real effect.
-
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