Network Master Operation
This section describes how the DL250–1 and DL260 can communicate on a Modbus or
DirectNET network as a master. For Modbus networks, it uses the Modbus RTU protocol,
which must be interpreted by all the slaves on the network. Both Modbus and DirectNET are
single master/multiple slave networks. The master is the only member of the network that can
initiate requests on the network. This section teaches you how to design the required ladder
logic for network master operation.
When using the DL250–1 or DL260 CPU
as the master station, you use simple RLL
instructions to initiate the requests. The
WX instruction initiates network write
operations, and the RX instruction initiates
network read operations. Before executing
either the WX or RX commands, we will
need to load data related to the read or
write operation onto the CPU’s
accumulator stack. When the WX or RX
instruction executes, it uses the information
on the stack combined with data in the
instruction box to completely define the
task, which goes to the port.
The following step-by-step procedure will provide the information necessary to set up your
ladder program to receive data from a network slave.
DL205 User Manual, 4th Edition, Rev. B
4–41
Chapter 4: System Design and Configuration
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A
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Slave #1 Slave #3
Master
Modbus RTU Protocol, or DirectNET
Slave #2
Slave
Master
WX (write)
RX (read)
Network
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