SLC 500 High Speed Counter Card

At my job on a specific machine the A+,A-,B+,B- inputs to a High Speed Counter Card are all connected in Series with Resistors. I don't remember the resistance value of the resistors but the moral of my question is, what are the resistors for?

Thanks

Your encoders are probably open-collector output types, and require "pull-up resistors" so that the when the transistor is OFF the voltage is "pulled up" to the 24v supply.

I am assuming here the resistors are not actually in series with the input, but are actually wired from the 24v supply to the input.

sometimes a series resistor is used to get the voltage level down--ie from 24vdc encoder pulse to 12 vdc counter input.

HSCE Series resistors

The 1746-HSCE was designed as a 5v Differential Line Receiver input. If your encoder has a higher level signal, 12v or 24v......the resistors act as current limiters, protecting the HSCE input circuit from high currents associated with the non-5V DLD inputs. The User's Manual for this card indicate the resistor size per the encoder signal voltage.