SEROUT being unreliable

[skimask]

Ok, i'm looking at the best way of multiplexing the hardware USART's output and this is what I have come up with.
- Use IC with 4 OR gates (only using 2, but they come with 4).
- Gate 1: Input1=USART, Input2=A PIC I/O Pin, Output=One of the serial outputs
- Gate 2: Input1=USART, Input2=Another PIC I/O Pin, Output=Another serial output
- Use software so that only 1 of the I/O pins is ever high at a time
I know I could use a NOT gate which would allow me to use only 1 I/O Pin, but i'd rather use another I/O pin than add another IC.
If anyone can see how this can be done better (and still only using the PIC + 1 IC), feel free to share.

That's exactly what I was talking about. If you wanted to get rid of the 7402 (?), you could easily do the same thing with a couple of 2N2222's.

[Kamikaze47]

That's exactly what I was talking about. If you wanted to get rid of the 7402 (?), you could easily do the same thing with a couple of 2N2222's.

I know how to use a transistor as an AND gate, but not sure how to do an OR. I need it to be OR because the output should be high when that output is not selected.

I've been trying to work out if I want TTL or CMOS for my OR gates. Ive been looking at the data sheets, but cant really see which would be better for this application.

[skimask]

I know how to use a transistor as an AND gate, but not sure how to do an OR. I need it to be OR because the output should be high when that output is not selected.

Then use the transistor it 'backwards'...(not backwards like C and E are reversed, just use inverted logic on it)
The output that is 'selected' isn't really 'selected'...it's 'unselected'...that way it idles high.
The other output that is actually 'selected' is really 'unselected'...passing the data thru.

If you use TTL, you'll be 'limited' to TTL voltages and switching levels (.8v low - 2.4v high roughly)
If you use CMOS, you'll be 'unlimited' to roughly 1/3 and 2/3 Vcc for your logic levels.
Depends on what you're driving at the outputs and what you're driving it with at the inputs.

But....your run of the mill 2N2222 will open up all sorts of options as far as voltage levels go. I suppose one of the problems with a transistor MIGHT be switching speeds. Go to fast, and the transistor won't switch fast enough (Early effect, Miller effect, etc). Sometimes it can be 'fixed' with a 'speedup' cap across base and emitter, sometimes not.

Again, depends on the final application.