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 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.
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.
DL205 User Manual, 4th Edition, Rev. B
4–44
Chapter 4: System Design and Configuration
1
2
3
4
5
6
7
8
9
10
11
12
13
14
A
B
C
D
Port Communication Error
LD
KF201
LD
K3
LDA
O40600
RX
Y0
SP116
Port Busy
SP117
SET
Y1
Interlocking Relay
LD
KF201
LD
K3
LDA
O40600
RX
Y0
SP116
SET
C100
C100
LD
KF201
LD
K3
LDA
O40400
WX
VY0
SP116
RST
C100
C100
Interlocking
Relay
