[Coco] serial port problem

[Gene Heskett]

On Thursday 31 October 2013 18:12:39 Tormod Volden did opine:

> On Thu, Oct 31, 2013 at 5:35 AM, Mathieu Bouchard wrote:
> > Hi, I have built an adaptor between a DB9 serial port and a DIN4
> > serial port, and I can send characters both ways at 300 baud and at
> > 2400 baud now, but when I try 4800, 9600 or 19200, I get either
> > garbage or nothing.
> > 
> > This is between Terminate 5.0 in MSDOS 6.22 with UART 16550, and a
> > CoCo 3 running UltimaTerm 4.1.
> > 
> > Any idea on how to fix this ?
> 
> I would have my doubts about UltimaTerm, especially after reading
> Aaron's reply. Or the PC side - have you tested higher speeds with
> something else than the CoCo, for instance loopback by shorting RX and
> TX on the RS232 plug? You could of course try loopback on the CoCo
> side as well.
> 
> Otherwise it could be hardware problem. Some CoCos (1/2 dunno about 3)
> have an operation amplifier (741) doing signal inversion on the TX
> line, and its poor bandwidth limits the baud rate. You could try
> exchanging it. Bad connections/soldering/isolation could also cause
> increasing problems at higher speeds but it would be strange that the
> result is so binary as you experience.
> 
> Tormod
> 
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The 741 is the most sloppily specified op-amp ever.  I have seen some that 
will make a slew rate limited sine wave out of a 1khz square wave.  Any of 
the ultra cheap lm357's is good to a megahertz or better, some to 10.  
Change it.

Cheers, Gene
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