Hi,

I'm just sketching up an interesting looking project at the moment
that some of you might be able to offer advice on.

A PC in a machine needs to control a whole heap of 24V DC
miniature compressed air solenoid valves. There can be up to 186
of these solenoids in one machine. Only a few valves will be on
simultaneously, and they draw about 300mA each. Worst case
scenario is 20% of the valves on at the same time. Prime
objectives are speed of control by the PC (minimum software
overhead and time delay before valve actuation.) There might be
1000 valve operations per second.

My ideas so far:-

- PC parallel printer port connected to PSP on PIC16F877 for fast
byte-wise transfer of data from the PC.

- PIC stores an image of valve data in RAM.

- PIC drives a whole bunch of TPIC6A595 8 bit serial input DMOS
power driver shift register IC's, connected in series, with clock and
data line from the micro's synchronous serial port. 20MHz PIC can
clock data at 1.25 or 5MHz.

- Might arrange the power shift registers in groups with a strobe line
per group to speed up the process.

My questions?:-

- Can the PC parallel port in a fancy mode like ECP or EPP or
something automatically generate a strobe pulse when 8 bits of
data are output? This would reduce the PC overhead.

- Has anyone used the TPIC6A595 from TI, or the A6A595 from
Allegro with good or bad comments, hints or tips? Is the built in
protection any good?

- Any comments welcome.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  brent.brown@clear.net.nz

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