DL06 Micro PLC User Manual; 3rd Edition Rev. E
4–21
Chapter 4: System Design and Configuration
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14
A
B
C
D
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2
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a
b
c
D
Interlocking Relay
LD
KF201
LD
K0003
LDA
O40600
RX
VY0
SP116
SET
C100
C100
LD
KF201
LD
K0003
LDA
O40400
WX
VY0
SP116
RST
C100
C100
Interlocking
Relay
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.
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
Port
Communication
Error
LD
KF201
LD
K0003
LDA
O40600
RX
Y0
SP116
Port Busy
SP117
SET
Y1