[Coco] Re: Supercomm: more details
John E. Malmberg
wb8tyw at qsl.net
Sun Aug 15 23:04:44 EDT 2004
KnudsenMJ at aol.com wrote:
> In a message dated 8/13/04 6:09:25 PM Eastern Daylight Time,
> james at skwirl.ca
>>As I see it, DCD should be fed by DTR and RTS...
As you point out, not RTS. You also do not want to tie two outputs
> I dunno, it seems that
>> DTR would be a sensible source for DCD... DCD is more of a
>> session-based on or off, like DTR and DSR (from what I've seen in the
>> past) whereas RTS and CTS are more for hardware handshaking...
Except for printers on a local connection. For some reason the printer
manufactures implemented DSR/DTR handshaking on them instead of RTS/CTS.
> You have a good point there -- DCD is indeed session-level, like DTR and DSR.
> However, many RS232 interfaces are designed to receive only when DCD is on,
> to prevent responding to noise on the line (makes plenty sense with a real
> modem), but I think the practice of feeding RTS to DCD on a Null modem was to make
> sure the receiving end read data only when the sender was actively sending it.
No, it was a bug because the person who said to do so did not know what
they were doing. That bug can be found in both hardware and software
that is mainly intended for use with PCs.
I have a HAM packet modem with that bug hardwired into it, and it
requires a b******d cable for me to connect it to use with standard
computer or terminal equipment. And it annoys me to have to keep track
of a non-standard cable because of someone not being able understand a
> ISTR some interfaces and software drivers are set up to drop the session the
> first time DCD goes down
It is a CCIT and equivalent requirement for a host computer session to
The DCD signal indicates that the link has valid data.
The DSR signal indicates that the communication hardware is online.
For an example of the difference between DCD and DSR is in communicating
with a standard modem to dial out from a computer, the computer asserts
DTR to the modem, and the modem answers with DSR with in a required time
The computer also optionally asserts RTS and gets a CTS response from
the modem, for the data handshaking. Most commercial equipment can be
configured by software to ignore the RTS/CTS lines as they are often
omitted from cables.
At this point, the computer can send commands to the modem to get it's
status, and issues dial commands.
When the remote system answers with it's carrier, the local modem sets
the DCD signal, this lets the computer know that there is a remote
If either the DSR or DCD drops, the computer assumes that the session is
gone, and it is supposed to drop DTR in response.
When the computer decides to terminate the link, it drops DTR. This
should cause the modem to drop the session, and reset it self to it's
power up defaults.
Some clones of the Hayes modem failed the test of reseting them self to
the power up defaults when DTR is dropped by the host. Which means that
they can be used for initiating dial out sessions, but can not be used
on a computer to answer incoming dial sessions.
> -- if so, you definitely do want DCD fed from DTR,
> not RTS.
Yes, if you want to be compliant with the official standards.
Personal Opinion Only
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