Re: Tesla Programmer
Yes, I had meant to mention to Chris, when I saw him earlier, that it might be best to have the 8255 removed in case it clamps these. But that makes testing for this more difficult, when it control the programmer and might be easier to just make an inline adaper to isolate the 4 data lines inputs to the 8255 (or carefully bend the relevant 8255 Input pins up, and re-insert IC into its socket).
However, I've just done a simple diode-check on an 8255, and there doesn't seem to be any forward-biased diodes / junction from port I/O lines to Vcc.
Although there is a reverse-biased diode / junction from Vcc to each of the port I/O's, reading around 0.6V - Which may be a consequence of TTL inputs, and these could have a reverse-breakdown (typically 7V for Vbe junction of silicon transistors?)
And there are also a slight reverse-biased diodes / junctions from the port lines pins to Gnd, measuring around 1.0V on DMM diode-test. - I don't think it's just internal resistance to ground, as you don't get any reading / > 2M on resistance range, reversing the probes.
So it doesn't seem that the 8255 would clamp these at 5V.
Although, if you tried an 82C55, you might well get different results, as I'd expect those to have clamp diodes on I/O pins for ESD protection etc.
|