The VME computer can communicate to the Allen-Bradley PLC through an A-B serial communications module located in an Allen-Bradley rack. The module is the 1785-KE allows the VME computer to act as a node on the data highway plus.
There are 5 layers in the serial communications of Allen Bradley PLC:
1. Physical Layer, there are three essential hardware elements needed for the PLC communications. They are:
• 1785-KE Module, It is a communications interface that links an intelligent RS-232-C asynchronous device
to an Allen-Bradley Highway Plus Network.
• VME Serial Port, Any serial port on a VME computer may be used to communicate with A-B 1785-KE module.
• Serial Cable, The RS-232-C serial cable connects the 1785-KE serial port.
2. Data Link Layer, Communications between the 1785-KE module and the VME serial port are through the RS-232-C protocol, including full duplex and hardware handshaking. There are two layers.
• Serial Port Driver
• EPICS drvSerial
3. Network Layer, The DF1 protocol provides the network, transport, and session layers of the OSI layers. They are including:
• EPICS drvAbDf1
• Writing to the PLC
• Reading from the PLC
4. Presentation Layer, the presentation layer of the PLC communications interface EPICS records with the network layer support provided by drvAbDf1.
• Common EPICS Record files
• Analog Input Records
• Analog Output Records
• Binary Inputs Record
• Binary Outputs Record
• Multi-bit Binary Input Records
• Multi-bit Binary Output Records
5. Application Layer, An EPICS database provides the application layer of the OSI model.