On Sun, Aug 04, 2002 at 03:53:55PM -0400, Byron A Jeff wrote:
> On Sun, Aug 04, 2002 at 09:41:20AM +0100, Kieren Johnstone wrote:
> > Thanks for your support (everyone!) :)
>
> You are quite welcome.
>
> > "Right, I'm gonna build TLVP!".. got both IC's datasheets, hooked it all up,
> > got to last connection, doh 74HCT125 only has 4 buffers :)  Gonna make
> > another order today/tomorrow to get that IC, probably some stripboard to put
> > the *working* programmer on aand probably a toy for me (7-segment digit or
> > similar).
>
> You can get away with only 4 buffers as the return connection of RB7/DATA
> back to pin 10 of the parallel port doesn't have to be buffered. Here's why:
>
> 1) The buffers are for level conversion from TTL to CMOS. The reverse
> connection is CMOS to TTL which is compatible.
>
> 2) The PIC is driving that line at full power (25ma).

And that's what I get when I send before I think. That connection must be
tested. Often the ACK pin on a parallel port (pin 10) will have a pullup
resistor attached to it. So you can get into a situation where the output
buffer from RB7/data won't have enough current to drive the pin low. So you
must carefully measure the low voltage after the resistor and make sure that
it's less than 0.5V. If not then lower the resistance until it is below 0.5V.
Do not go below 300 ohms. If at 300 ohm the voltage when low is still above
0.5V then wait until your buffers show up.

BAJ

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