Use cRIO DO module NI 9477 as relay to provide 24V input for Siemens S7-200 (CPU222)

[jinyu0315]

Dear All, could anyone please confirm my usage of NI 9477 as a relay works or not with following details. It might be too silly question but I am sure it is not posted before and I really cannot afford to burn my modules. I have two worries, one is sink or source connection, another is overcurrent issue.

The Siemens S7-200 is used to control a step motor driver by sending pulses. I need to provide two digital signals to control step motor's rotation direction. So I propose to connect my NI 9477 as a relay switch shown in the graph below (left configuration).

According to the NI 9477 datasheet (attached), I suppose I should connect it as the right configuration as NI 9477 is sinking module. However, current power supply in the control cabinet is wired as left configuration. I am not sure if I can just connect my NI 9477 or I need to adjust the power supply polarity. 

Another concern would be the current through NI 9477. It rated at 1A and will be connected in series with only a 24VDC power and s7-200 digital input. I am not sure if the current would be a concern as in s7-200 datasheet, it seems that the current is only 4mA. Am I reading this correctly? I also attached the s7-200 datasheet if you have time to varify that, please.

Thanks in advance for any help.

Regards,

Jinyu

[crossrulz · Accepted solution]

Personally, I would just change over to a 9476.  The 9477 looks like it is best for negative logic (24V = FALSE, 0V = TRUE).

And current for the PLC is practically in the noise for a 24V digital signal, so I would not worry about it.

[jinyu0315]

Dear crossrulz,

Thank you very much for your fast and professional suggestion. Now the overcurrent problem has gone.

For the module change, it is not practical at this time as I have already purchased the 9477 with the reason that each channel can take more current (1A) over 350mA (roughly, I cannot remember) on 9476. 

For the time being, could you please suggest if 9477 can be used in my case for now?

Regards,

Jinyu