CPU Specifications
and Operation
4–44
CPU Specifications and Operation
Typically network communications will
last longer than 1 scan. The program must
wait for the communications to finish
before starting the next transaction.
Port Communication Error
LD
KF201
LD
K0003
LDA
O40600
RX
Y0
SP116
Port Busy
SP117
SET
Y1
Port 2, which can be a master, has two Special Relay contacts associated with it (see
Appendix D for comm port special relays).One indicates “Port busy”(SP116), and
the other indicates ”Port Communication Error”(SP117). The example above 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.
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, C0 is set. When
the port has finished the communication
task, the second routine is executed and
C0 is reset.
If you’re using RLL
PLUS
Stage
Programing, 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.
Interlocking Relay
LD
KF201
LD
K0003
LDA
O40600
RX
Y0
SP116
SET
C100
C100
LD
KF201
LD
K0003
LDA
O40400
WX
Y0
SP116
RST
C100
C100
Interlocking
Relay
Communications
from a
Ladder Program
Multiple Read and
Write Interlocks
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