--- In Jason Stahls <> wrote:
>
> Doug Chapman wrote:
> > Sending 3.3v to the 5v device is not a problem as that is enough
to trigger the logic on the 5v device. The problem is the 5V device
sending a signal to the 3.3v TX7400. I am wondering if for 5V ->
3.3V if just adding a current limiting resistor would suffice in
keeping the board safe?
>
>
> Toss a couple transistors at the problem. From TX 3.3v to RX 5v use
a
> NPN with the emitter to ground, collector to 5v RX tied high via a
> resistor to5v. For TX 5v to RX 3.3v do the same just tie the
resistor
> to 3.3v instead of 5v. You can then dump that through a inverting
> buffer or another transistor setup to invert again as the first
> transistor will invert your data.
>
> <Caution, bad ASCII art to follow>
> +5v
> |
> \
> / R4
> +5v \
> | /
> \ |--------------- RX
> / R2 |
> \ | / C
> / R3 B |/
> |-------/\/\/\-----|
> | |\
> | / C | \ E
> R1 B |/ |
> TX --/\/\/\- --| GND
> |\
> | \ E
> |
> GND
>
> I'm not 100% sure of the resistor values as they depend on the
> transistor your using (and I'm too tired right now to figure them
out
> anyways) R4 should be good around 4.7ish K, I think R1/R3 would be
ok
> at 10-12k. YMMV, don't blame me if you don't confirm my #'s and
blow
> something up :)
>
> Jason Stahls
>
Hi,
Another solution is to use the new 74lvc series logic. This runs off
3.3v but all inputs are 5.5v tolerant. I believe they are designed
specifically for this very purpose.
see http://www-origin.nxp.com/pip/74LVC1G08_7.html
Cheers Noel Diviney.
------------------------------------
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/ts-7000/
<*> Your email settings:
Individual Email | Traditional
<*> To change settings online go to:
http://groups.yahoo.com/group/ts-7000/join
(Yahoo! ID required)
<*> To change settings via email:
<*> To unsubscribe from this group, send an email to:
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
|