Well, this is a very generic question. There are hundreds of communication protocols used by hundreds of different PLC types. Additionally there are lot's of physical communication layers that could be used. This means that there is no single simple answer for your question.
In general there are several ways for communicating between FieldPoint and PLCs. Here are some options:
Basic I/O communication:This is very straight forward. Use 24 V digital input and output modules on both, the PLC and FP and communicate through digital I/Os. This is ok for small distances and simple communication tasks like handshaking, but it's not scalable for more complex applications.
Direct serial port or ethernet communication:You can use the communication interfaces on an embedded FieldPoint controller (RS232, RS485, Ethernet) to communicate with PLCs. There are also some higher level communication protocols available in LabVIEW like Modbus and Modbus TCP. If you need to use a protocol that is not available for LabVIEW Real-Time, yet, you will have to program the protocol by yourself, which requires good documentation and might be sometimes tedious or for very complex protocols even impossible, as you couldn't reach the required timing without a dedicated CPU for the protocol.
Fieldbus gateways:Another option is using a serial or ethernet fieldbus gateway. These devices handle the communication protocol and provide a fieldbus interface to the PLC and a simple interface to the communication partner - in your case to FieldPoint through ethernet or RS232.
Here is one example of these devices. With this solution you will still have to implement a protocol to talk to the gateway, but this is typically quite easy.
I hope this helps,
Jochen Klier
National Instruments
