when do you select TTL Sourcin/ Sinking Input/Outputs card

Can someone kindly explain, when do i select a TTL Sourcing or Sinking Input/ Output module over other module.

Thanks in advance.

I pick my cards based on what external hardware I have. IE...is the sensor PNP or NPN.

IF the sensor is a NPN then I will need sourcing cards
If the sensor is PNP then I will need sinking cards.

Output Card are decided the same way (well I decide the issue the same way) Does my device require a sinking our sourcing signal.Have worked with VFD drives that required a sinking signal from the PLC to operate.

If worst comes to worst and you pick the wrong card you can install interposing relays are use a pull up resistor (personally I would change the device or the card)

Thanks Clay

but my question was why TTL??

TTL as I understand it means transistor to transistor logic.

I have heard people refer to a ttl card when needing a card that operated in the 5 vdc range. I know that AB lists there 5VDC cards as TTL cards.

Hope that helps

As far as I know, TTL output is always sinking.
Why use TTL input/outputs? To work with TTL level signals which are incompatible with typical PLC DC inputs.
For instance, now discontinued Omron barcode reader sensor V500something had Match/No Match TTL outputs.

Some clarification: actually, TTL is a voltage output with sinking active level of signal.
Not to be mixed with sinking open collector, line driver and tri-state 5V rated outputs.