Standardization is the main problem. I have heard that one motherboard
manufacturer designed his parallel ports tri-stating the data lines in
between transfers. The dongle people didn't like this implementation at all.

I have one of those parallel port switches and it derives power from the
port. It's called PriMax and I purchased it at Office Depot. It does provide
for a 6 volt input, just in case.

More parallel port designs are becoming IEEE 1284 compatible/compliant and
the voltage levels and the current available that you might expect may be
different due to the way the drivers are implemented and the way the lines
are terminated. You can expect to see series resistance on the drivers for
the data lines and the strobe line. The receivers have both pull-up and
pull-down resistors.

I have been able to run a PIC solution to 115,200 bps and with the newer
Maxim chips I suspect I could reach 230,400 bps.  That's 23,000 characters
per second (if the PIC overhead would allow it). The advantage here is
serial comm controls for the PC app are readily available, cheap and well
tested over different operating systems.

The IEEE Std 1284-1994 IEEE Standard Signaling Method for a Bidirectional
Parallel Peripheral Interface for Personal Computers ISBN 1-55937-427-6 is
available at IEEE in NY. You can order a copy by calling 800-678-IEEE in the
US, 908-981-1393 outside the US and Canada.

Rich


At 06:41 PM 2/28/98 GMT, you wrote:
>Anyone got any (bad) experiences of making PC parallel port gadgets
>with a PIC in ?
>I need to do a box to squirt out data in a proprietary serial format,
>and the PC serial port isn't quite fast enough.
>The interface will be pretty much like a printer - strobe data out,
>ack and/or busy to acknowledge, probably one other line (probably
>paper out) for status back from the PIC.
>I want to power the PIC from the port, which should be good for the
>1mA or so needed at >4V.
>I'm planning to take power from a couple of control lines, and a
>couple of data lines via schottky diodes into a 100u reservoir cap.
>When I did something similar a long while ago, I found that some ports
>put out more current on the data lines & some on the control lines, so
>I'm hedging bets!
>The PC S/W will be Turbo Pascal bashing the port directly in 'vanilla'
>unidirectional mode, polled ( i.e. without interrupts), and I'll
>arrange for the data lines to be high when not transferring data to
>keep the power going.
>
>The design & software is no problem, but there are so many different
>flavours of PC parallel port out there that  I'm  looking for info on
>any 'nasties' to watch out for - machines with unusual ports/strange
>levels etc. Has anyone come across ports which don't have pullups on
>the data/strobe lines ?
>
>
>     ____                                                           ____
>   _/ L_/  Mike Harrison / White Wing Logic / wwl@netcomuk.co.uk  _/ L_/
> _/ W_/  Hardware & Software design / PCB Design / Consultancy  _/ W_/
>/_W_/  Industrial / Computer Peripherals / Hazardous Area      /_W_/
>
>

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