question

plz tell me the logic level of PLCs and let me know how to acheive an interface between PLC's logic level and TTL logic level...

thanks

Most modular PLCs offer TTL level input and output modules.

The logic level of the PLC internally is probably 5VDC. However, this doesn't matter. The purpose of the I/O cards is to interface the "real world" voltage levels to the PLC logic level. All you need to do is match the characteristics of the field device voltage to the rating of the input card, and the card will do the rest. For example, if you have 230 VAC power through an external switch, using an input card rated for 230 VAC will accept the voltage and convert it to an "ON" status at the input card. If you have a 5 VDC TTL field device, use an input card rated for 5 VDC. The card will accept voltage at that level and convert it to an "ON" status for the PLC.