Ethernet Communications Modules, 3rd Edition, Rev. D
5-21
Chapter 5: MODBUS TCP for H0/H2/H4-ECOM100
1
2
3
4
5
6
7
8
9
10
11
12
13
14
A
B
C
D
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.
Depending on which slot the ECOM is in, it has two
Special Relay contacts associated with it (see page 4-11
to 4-12 for special relays). One indicates “Port busy”,
and the other indicates “Port Communication Error”.
The example at right shows the use of these contacts for
an ECOM that is in slot 1. 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 it will be
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 your are 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.
LD
K114
LD
K3
LDA
O40600
RX
V0
SP122
ECOM Port Busy
SP123
SET
Y1
ECOM Communication
Error
ECOM Port Busy
Interlocking Relay
LD
K114
LD
K3
LDA
O40600
RX
V0
SP122
SET
C100
C100
LD
K114
LD
K3
LDA
O40400
WX
V0
SP122
RST
C100
C100
Interlocking
Relay
