[Coco] Null modem cables?

[Frank Swygert]

Gene, I don't recall the details, but I think you're right -- the 
DTR/CTS either need crossing or connecting together. I seem to recall 
three lines all being connected for a cable, but that might be the 
bit-banger and not the RS-232 pak. The info for a bit-banger cable is in 
my book (Tandy's Little Wonder, d/l from the list site in farna folder), 
and that might help.

----------------
Date: Mon, 22 Feb 2010 09:41:01 -0500
From: Gene Heskett <gene.heskett at verizon.net>
> 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)
> where
> 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.
>
>   


-- 
Frank Swygert
Publisher, "American Motors Cars" 
Magazine (AMC)
For all AMC enthusiasts
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