Quote:
Originally Posted by Mark1960
When Vcc is raised to 10.5 volts does the output voltage of unprogrammed data pins also increase to 10.5v? This might be clamped to 5v by the esd protection diodes internal to the 8255, so just wondered if current limit resistors should be added on the 8255 inputs.
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IIRC, Most PROM's default to logic-0 when unprogrammed / fuse not 'blown' - The opposite to EPROM's that default to all 1's when erased.
However, I'm not sure what happens once programmed, and Vcc is still at 10.5V.
- Especially as programming on these Tesla ones seems to be inverted, with having to pull data-lines to 0V to blow the fuse to make output '1'. So if the current-sink transistor is switched-off, whilst Vcc is still at 10.5V, then maybe the data-line will go to that for a while with standard '3-state' output (Rather than Open-collector) types.
However, if these PROM's are like other 74(S) TTL, then output source current may be rather low, and so 8255 Input protection diodes (If present on non CMOS types?) should clamp it without lifting it's supply rail too much.
From bitter experience of having two pairs of 8255's fail, when designing my own Acorn BBC computer-controlled universal EPROM programmer > 30yrs ago, I discovered that having too much capacitance on the EPROM Vpp pin and simply relay-switching this back to the +5V rail, resulted in that being lifted (as 7805's etc can only-source) to a high-voltage for a short period, which killed the 8255's!
- So I re-designed it, to have Vpp supply totally-independent of the main +5V supply to the control IC's, and have Vpp settable to +5V by itself.
I'll mention it to Chris, to see if he's looked into the possibility of the above issue / may be able to do some 'scope checks whilst programming a test IC.