Communications From A Ladder Program; Multiple Read And Write Interlocks - Automationdirect.com DL05 User Manual

Communications from a Ladder Program

Typically network communications
will last longer than 1 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 (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.

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, C0 is set. When the port
has finished the communication task, the second
routine is executed and C0 is reset.
Chapter 4: Configuration and Connections
Port Communication Error
DL05 Micro PLC User Manual, 6th Edition, Rev. D
SP117
SP116
Port Busy
Interlocking Relay
SP116 C100
Interlocking
Relay
SP116 C100
1 1
2 2
3 3
Y1
SET
4 4
LD
KF201
LD
5 5
K0003
LDA
O40600
6 6
RX
Y0
7 7
8 8
9 9
LD
KF201
10 10
LD
K0003
11 11
LDA
O40600
12 12
RX
VX0
13 13
C100
SET
14 14
LD
A a
KF201
LD
B b
K0003
LDA
c C
O40400
WX
D D
VY0
C100
RST
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