Ethernet Communications Modules, 3rd Edition, Rev. D
4–2
Chapter 4: RLL Programming for Communications
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PLC-to-PLC Communications
This chapter steps you through the development of a Relay Ladder Logic (RLL) program
to enable one PLC to communicate with another PLC. For the experienced programmer of
DirectLOGIC PLCs, the communication programs presented in this chapter will be simple
to follow. If you have never programmed a DirectLOGIC PLC, you may want to refer to
the DirectSOFT Programming Software User Manual and the User Manual for your PLC for
additional information.
NOTE: The programs described in this chapter are not used for communication between a PC and a PLC.
For PC-to-PLC communications, please see the product documentation for the PC software you are using. If
you are using our DSData Server software, the manual you will need is the KEPDirect for PLCs User Manual.
How RLL is Used for Communications
DirectSOFT Programming Software provides Read and Write instructions (RX/WX) for PLC-
to-PLC communication over a network. The Read and Write instructions are part of the ladder
logic program running in the CPU of the initiating, or master, PLC. These instructions tell the
initiating CPU to send a message over the Ethernet network to a responding, or slave, PLC.
The ECOM module is the connecting point to the network for each PLC. The initiating PLC’s
Read or Write communication finds its destination by the Module ID of the responding PLC’s
ECOM module. See Chapter 2 and 3 for information about assigning Module IDs.
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