Hi,
> Yes, I want to interface between 3.3V TTL UART and 5V TTL
> UART without using any RS232 shifter to reduce hardware
> parts. I am only concern with 3 lines, TX, RX and GND.
Ok.
> In fact, my 5V TTL sensor can receive data possibly because
> the 3.3V logic output still meet 5V TTL input requirements,
> but not the other way round, that's why I cannot receive
> the data out from 5V sensor output into the TS7800 3.3V
> TTL UART. I think I just need to meddle with this RX line
> and leave the TX line untouched.
You need to be careful when interfacing logic that is
powered from separate power supplies.
It is highly likely that your 3.3V TS-7800 transmitter can
interface directly to the 5V UART. However, if that 5V
UART has its power *off*, then the TS-7800 output could
be damaged (depending on what happens when you drive the
unpowered 5V UART).
> I also read in maxim website that simple resitor voltage
> divider with diode can help to do that...but I just want
> to verify by calculting if the resistors used will be
> compatible to my interface...
You can certainly resistively divide the 5V transmitter
signal down to 3.3V to drive onto the TS-7800. However,
you will again be faced with the issue of what happens
when the TS7800 is off, and the 5V UART is driving its
pins.
Using a buffer between the devices is safest, as they
are design to be 5V tolerant regardless of whether they
are powered or not:
http://www.fairchildsemi.com/ds/NC/NC7SZ125.pdf
"The inputs and output are high impedance above ground
when VCC is 0V. Inputs tolerate voltages up to 6V
independent of VCC operating voltage."
If you want to do a foolproof job, then use buffers on
the TX/RX lines.
However, if you are just trying to get something working,
put a 1k resistor on the 3.3V transmitter output to the
5V receiver input, and that'll limit current in the
case the 5V UART is off, and put a voltage divider from
the 5V UART output to the 3.3V input. Eg. a 1k+1k divider
will take a 5V level signal to 2.5V. That'll be ok to
start with. You can measure the voltages and adjust
accordingly.
Cheers,
Dave
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