[Coco] Null modem cables?
gene.heskett at verizon.net
Mon Feb 22 09:41:01 EST 2010
I am about to throw in the towel in making serial comm work between this linx
box and my rs-232 pack.
I have now pulled the connector off the coco end of the cable, and applied a
pulser to the cable hooked up here to identify the signals, went down and
hook the wire with the pulse on it back into the correct solder cup on the
back of the db25 plug so as to make a null modem cable as per
http://www.camireasearch.com/Data_Com_Basics_(there is more but its the
downloadable page on the rs-232 std.) which I printed, and what I have
matches the hookup described as "10 - DB25 NULL MODEM (standard handshaking)
pin 1 & 7 are grounds, 7 specifically is signal ground
Pin 2 on 1 end is connected to pin 3 on the other end and vice versa as the
RD/TD data lines. Normal, works.
pin 4, RTS <--pin 5 CTS and vice versa
Pin 6 & 8 are DSR & CD connected to pin 20 as DTR
So when the buffer is full on the coco end it drops DTR, which is seen here
as DSR & CD turning red on the led sniffer.
Now, I think this DTR really should be connected to CTS, not DSR & CD.
The symptoms of course are that this linux box ignores what should be the
stop signal, and just keeps pouring data into the pipe. So what should be a
740 cps zmodem transfer while running at 9600 or 19,200 baud (rz being the
limiting factor on a 6309 equipt coco3, and its in the high 400's for a 6809
coco3) degrades to about 150 cps due to all the error correction backups and
resends sz has to do.
This drain bamaged 6551 we are stuck with can't be made to drop any other
signal but DTR, or RTS (a transmitter function) and it can send a long break
on the data line, which IMO is not the proper way to do it.
So the question is this:
Does this work for you folks using a std, off the shelf null modem, or have
you been forced to invent your own flow controlling null modem?
And if so, how did you wire yours up? I'm inclined to cross couple DTR and
CTS instead, but this is not the adapter you can buy, you would have to make
it yourself. To me it even makes more sense, but...
Thanks for any insight you can share.
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-Ed Howdershelt (Author)
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