[Coco] DriveWire issues with Coco 1 F/285 Board

[Gene Heskett]

On Monday 01 July 2013 05:53:55 Torsten Dittel did opine:

> I remember darkly an issue my father had on an early (below F) CoCo1
> board: I was playing with MIDI output (31.25kBaud - e.g. like that of
> LYRA) via the bit banger port. It didn't work reliable on a CoCo1 while
> it did on my CoCo2. When we attached a scope, we figured out that slew
> rate (both rising and falling) of the signal was to small, so single
> bits (0 or 1) turned to be triangles with reduced amplitude instead of
> the expected rects (with full amplitude). That was sometimes catched
> correctly by the receiver (in that case: MIDI keyboard), sometimes not.
> I remeber my father replaced a component with faster response time. As
> far as I remeber, the output pin 3 (?) of the corresponding PIA is tied
> to a 741C inverter op-amp. I guess it's just replacing that one (I
> think it was even socketed on the PCB) with a current version, but you
> might want to wait for some comments of the hardware gurus out here.
> 
> Regards,
> Torsten

The 741C is a very slow op-amp, incapable of doing more than a low 
amplitude, slew rate limited triangular wave at 31.25 kikobaud.  Its full 
gain bandwidth is only 10Hz!  Even if you have to re-arrenge the basing, 
one of the op-amps in the LM35X family would be many many times more 
capable a device.  The LM358, in an 8 pin minidip, actually contains 2 
fully compensated devices with .1 megacycle plus abilities at greatly 
reduced power consumption.
> 
> 
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Cheers, Gene
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