[Coco] Printing from DW

[Gene Heskett]

On Monday 27 October 2014 11:35:47 Al Hartman did opine
And Gene did reply:
> In "Turn of the Screw" there is a Parallel Port Cartridge.
> 
> If someone made up the board for it, I'd love to build a couple.
> 
> Couldn't an OS9/NitrOS9 driver be written for it, and then the printing
> problem would be solved. He already provides patches for DECB.
> 
> -[ Al ]-

Someplace in my midden heap of code is a very simple driver for the 
parport on the side of either the 4n1, or J&M-CP controllers.  I can post 
it if I can find it, probably on my web page.

But it has one very glaring problem.  When the FCC mandated in the mid 
80's the radiated noise level controls, most printers including our 
miserable but beloved DMP's, had enough filtering applied to their parport 
inputs that the only way to actually write to a parallel printer would 
have been to add a timer circuit someplace in the controller that was 
triggered by the /cs at the port pin called strobe, and which fired off 
pulling the halt line back to the 6x09 low while the data strobe was 
active, thereby freezing the 6x09 in the write cycle for 5 to 10 
microseconds, giving the data time to get thru the filters and into the 
printer.

I have both controllers, but never managed to find a suitable round tuit.  
Then I got a decent serial printer and never looked back.

But now all you hardware guru's know what you have to do to make it work 
well.

There is a second alternative too, and that would be to add an 8 bit wide 
latch chip (74HCT245?) in the cable, again with a timer to hold the data 
stable for a few microseconds and then clear the strobe back high until 
the next byte is written. There may be enough lag between bytes sent that 
no time killers would be needed in the driver. That would be the 
preferable method in fact. That has the slight disadvantage of needing a 
few milliamps of 5 volts from someplace.  Schottky diodes off the data 
lines could probably steal enough power for HC chips.  The printers inputs 
would be the major load.

Cheers, Gene Heskett
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US V Castleman, SCOTUS, Mar 2014 is grounds for Impeaching SCOTUS