Re: [ts-7000] Re: TS7400 serial ports - 5V or 3.3V?

[Jason Stahls <>]

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

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