SN75189A - response control

Paul,

May I assume that this is not for a RS232 application?
What are your expected low and high input voltages?

Hi Ron,

Thanks for the very quick answer!

Indeed as you said, not exactly a RS232 application of the SN75189A :) I intend to use it for 24V "logic" inputs, and what I intended with modifying the threshold level is to improve the noise margin for a (theoretical) 0V input. 

Paul,

I was thinking of a series resistor between the 24V logic and the input A pin to decrease the voltage seen by the SN75189A to protect it. This will also increase the threshold (relative to logic voltage).

Do you have any negative power supplies the system?  Response control resistor with negative voltage is needed to increase threshold further.

Hi Ron,

I have the 24V logic input (in most cases it will be a open-collector NPN/PNP configuration), a pullup/pulldown to 24V and 1K in series with the SN75189A input. Unfortunately I have no negative supply available.

Paul,

Without a negative pull down, getting a large threshold voltage won't be easy.
Using standard comparators with some ESD/EOS protection on the inputs may be a better choice.
What is the data rate needed?

Hi,

I ran a quick SPICE simulation based on the schematics in the datasheet, and with 220R ot GND from the RC pin and 1K series with the input, the threshold seems to be ~16V. Which is more than satisfactory, if the schematics & sim are correct. Am I missing something? 

The data rate is not really an issue, these will be 24V logic inputs from some sensors / switches. 

Regards!