DL205 User Manual, 4th Edition, Rev. D
4-44
Chapter 4: System Design and Configuration
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Communications from a Ladder Program
Typically, network communications will last
longer than one scan. The program must
wait for the communications to finish before
starting the next transaction.
Port 2, which can be a master, has two
Special Relay contacts associated with it.
One indicates “Port busy”(SP116), and
the other indicates ”Port Communication
Error”(SP117). The example shows the use
of these contacts for a network master that
only reads a device (RX). The “Port Busy”
bit is on while the PLC communicates with
the slave. When the bit is off, the program
can initiate the next network request.
The “Port Communication Error” bit turns
on when the PLC has detected an error. Use
of this bit is optional. When used, it should be ahead of any network instruction boxes since
the error bit is reset when an RX or WX instruction is executed.
Multiple Read and Write Interlocks
If you are using multiple reads and writes in the RLL
program, you have to interlock the routines to make
sure all the routines are executed. If you don’t use the
interlocks, then the CPU will only execute the first
routine. This is because each port can only handle
one transaction at a time.
In the example to the right, after the RX instruction
is executed, C100 is set. When the port has finished
the communication task, the second routine is
executed and C100 is reset.
If you’re using RLL
PLUS
Stage Programming, you
can put each routine in a separate program stage to
ensure proper execution and switch from stage to
stage allowing only one of them to be active at a time.
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