Is this (electronically) legal?

[mstechca]

I am trying to build my own ROM programmer for the 28C64 EEPROM.

However, I have one question.

In order to avoid using an OR gate or 2 diodes, I was wondering if it is electrically safe to connect two outputs and one input together.

The outputs are 1 output from a CMOS 4081 AND gate, and one output from the CMOS 4017 counter. No more than 5V is flowing through the entire circuit. The input is the enable pin from the counter.

Why do I have this setup? because there are two things that can prevent my counter from moving on to the next stage. One is when the counter reaches the end, and the other is when the ROM is in read mode, and valid data isn't received.

I can understand if I need to waste a chip for an XOR or an XNOR gate, but it is ridiculous to use one for an OR gate, if I don't have to.

And I was wondering if I could get away with a 200uS timer.
In other words, If I set the "WE" pin low for 200uS, then the "WE" pin high for 200uS, and then the "OE" pin low and left "OE" low, will the data I/O pins eventually return whatever data I fed it when "WE" was low? or is it more complex?

 

[Nigel Goodwin]

You can't normally join two outputs together, ONLY if they are open-collector, in which case it forms an OR gate.

 

[evandude]

yes, really, what do you EXPECT to happen when one output is high and one is low, and they're directly connected?

and why is adding a couple of diodes and a pull-down resistor really that big of a problem?

 

[Nigel Goodwin]

and why is adding a couple of diodes and a pull-down resistor really that big of a problem?

Because he'll get it wrong? :lol: :lol: :lol: :lol: :lol: :lol: :lol:

 

[mstechca]

and why is adding a couple of diodes and a pull-down resistor really that big of a problem?

Because he'll get it wrong? :lol: :lol: :lol: :lol: :lol: :lol: :lol:

No, because I want to drill fewer holes :lol:

You can't normally join two outputs together, ONLY if they are open-collector, in which case it forms an OR gate.

I redid my circuit so that no two outputs join together.

 

[Hero999]

Ah,it's a tough life - having to drill a few extra holes.

 

[evandude]

No, because I want to drill fewer holes

I feel the same way, that's why I use mostly surface-mount parts... once you get the hang of it, it's faster to solder parts, takes up less board space, and requires less drilling, and no lead clipping... and the parts are often cheaper as well.

oh wait, I forgot, you only buy parts locally, so you probably can't get a lot of surface-mounts :roll:

but seriously, I have caught myself spending way too much time trying to work my way around having a couple of jumpers or a couple extra parts. But think about it... in the time it took you to ask this question on the forums, you could have drilled the holes for a couple diodes and a resistor 10 times over. often it's really just easier to go the straightforward route, and usually works better too :lol:

 

[JohnBrown]

You can't normally join two outputs together, ONLY if they are open-collector, in which case it forms an OR gate.

Not in the case of NPN transistors it doesn't!

 

[Nigel Goodwin]

You can't normally join two outputs together, ONLY if they are open-collector, in which case it forms an OR gate.

Not in the case of NPN transistors it doesn't!

Depends on your system of logic I suppose, and what's actually in front of the open-collector output, but a NOR gate if you like?.

 

[falmeshon]

mstechca-- what programming algorithm are you using out of interest?

 

[Nigel Goodwin]

mstechca-- what programming algorithm are you using out of interest?

From what I can gather, he's doing it COMPLETELY in hardware! - I hate to think what he's actually doing?.

 

[Hero999]

You can also surface mount through hole componants if you're too lazy to drill the holes.

 

[mstechca]

mstechca-- what programming algorithm are you using out of interest?

From what I can gather, he's doing it COMPLETELY in hardware!

Yes, mostly. The hardware (2 4040 counters) controls the address lines, and the software controls what data goes in, the counter increment, the counter reset, the WE pin, and the OE pin of the EEPROM is controlled by the PC.

There are three statuses coming out of the programmer. One is a low value indicating the programmer is connected, the second is a "rom full status", and this is high when the last byte is written on the EEPROM chip, and no more bytes could be added. The last is the bit check status. The EEPROM is read and compared to the data from the parallel port. It only checks one bit, and the output is based on whether the bits are the same or not.

I hate to think what he's actually doing?.

:lol:
perhaps you may like what I am doing.

The circuit works, and it will continue to work.
It only uses 5V!

 

[neon]

only 5volts good luck.

 

[akg]

perhaps you may like what I am doing.

and what r u doing..?