Serial interface of the UIRT

[Kay]

Hi,
can anybody explain me the serial interface of the UIRT. Normally people uses Max232 or others for this. Here we have a direct connection from RxD,Cts with the PIC. I'm wondering because it's works.
Remember the Pins of PIC are TTL-Pins.

best regards
Kay

[andybryant9]

Hi Kay,

The max232 inputs are actually quite lax in their max/min needs. So even though the spec says 12V, 5V is usually enough.

Andy.

[Kay]

Hi Kay,

The max232 inputs are actually quite lax in their max/min needs. So even though the spec says 12V, 5V is usually enough.

Andy.


Oh no, on RS232 you find positiv for High and negativ for Low. This is not TTL.
I'm wondering, that the circuit works without destroy anything.
Now I'm working of a redesign.

best regards
Kay

[Luc]

The official specs for RS232 are:
A high level must be in the range from +3 to +12 and a low between -3 and -12, everything between +3 and -3 is considered undefined.
In practice however the the RS232 receiver chips produce a high for everything above +2v and a low for everything below +2V.
Thus in reality RS232 uses the same 2V level to decide low or high.
Care must be taken to protect the PIC inputs but with a series resitor and the internal clamping diodes to ground and +5V the PIC is adequately protected.

[Kay]

O.K. Luc,
but I can't see a shunt on schematic.
Here we have a direct connection from RxD,Cts with the PIC !
Or, is this not the actuell schema:
http://users.skynet.be/sky50985/uirt2a_schem.pdf

ciao
Kay

[Luc]

The resistor is only required to reduce the level from the RS232 outputs and thus protect the pic inputs.
RXD and CTS are RS232 inputs and don't need protection, they receive from 0 to 5V from the pic output.

[Kay]

Thank you Luc,
I mix up RxD and TxD.