On Thu, 9 Nov 2000 23:09:45 +1300, you wrote:

>Thanks everyone for the replies. I'll pass these comments about
>Windoze on to my client. He does have an existing product
>working though, driving the valves directly from an ISA card. Makes
>me wonder how good it actually works. I can see that variations of
>several milliseconds would cause imperfect operation of the
>machine.
>
>I know that a solenoid valve won't respond in 250us, but I guess the
>idea we are working on here is trying to keep all the variables at
>least one order of magnitude below that where they start causing
>concern. I'll ask a few more questions.
>
>The EPP port looks quite cool now (a bit scary at first), lots of good
>web sites around for info. At first look the PSP port doesn't seem
>to have the right kind of logic to make it suitable for this use. I
>wondered about just an 8 bit port and an interrupt line, but I'm not
>sure if handling nested interrupts is feasible on the PIC (eg.
>allowing for EPP interrupt to occur during a timer interrupt or MSSP
>interrupt). I'll persist with the PSP a bit more.
You can't do nested ints, but you don't really need to. It seems you
basically have 2 tasks - pc comms and timed operation of the
solenoids. Only one of these tasks (the most time-critical) needs to
run on ints - probably the PC comms. The timer stuff can run polled in
the foreground - the few microseconds' jitter caused by doing this
should not be an issue in this appliction.
If you can make your interface look like a printer to the PC this will
certainly simplify things - this can be easily done on the PIC with an
interrupt line from /strobe. I believe one feature of the various
enhanced parallel ports on PCs is a transmit FIFO - if you pretend to
be a printer you can make use of this functionality to ease the timing
burden on the PC. It would also allow more hardware flexibility, and
be likely to work on different PCs. =20

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