[dorkbotpdx-blabber] Designing a Parallel-to-USB Converter

[Tim Hatch]

> 1) I see an inverter on the strobe line of the parallel port.  I  
> don't see
> it on the eagle layout.  Maybe I'm looking at the wrong one.

It's a single-gate version that looks like an SOT-23, "U$2".  In the  
first layout, it's between the 74374 and the tac switches; in the  
second on the upper-right.

> 2) You're using an edge triggered device (the 374) for a parallel port
> application. You said you had it prototyped.  Try dropping in a 373  
> instead
> and take out the inverter (74LS04) on the strobe line.  The 373 is  
> level
> triggered.

Will do.  I had the 373 in there at one point, but googling suggested  
the 374 and I didn't try building with the 373.  I'll give it a shot,  
as well as Don's no-latch suggestion.

> 3) Depending on what your final application is, some software (none  
> I ever
> wrote.  BAD PROGRAMMER, BAD!) *will* actually use the NACK and BUSY  
> signals.
> You might tie them into unused ports on the AVR chip and that way in  
> the
> future you could implement them in software if needed.

Ouch.  You're right, I forgot to draw in lines that I'm actually using  
right now to the schematic.  In addition to NACK and BUSY, it's  
essential that I be able to control ERROR/Paper-Out.  I've uploaded a  
new version at the same URL, which in addition to fixing all the  
missing lines (including strobe, which was supposed to go to an  
interrupt pin all along) uses the 74*373, tiny switches and adjusts  
the width to what I actually have in the connector.

> 4) No support for bidirectional parallel ports??  SHAME!  :)

Of the dozen parallel devices I have seen or we have at the office,  
all of them use compatibility (centronics) mode.  It seems that points  
of sale intend to use compatibility mode when talking to thermal  
printers, and they _do_ pay attention to the paper out line (tried it  
manually).

Tim