User guide

Instructions
Chapter 18
I/O Message and Communication
18–10
To view a MSG instruction error code when troubleshooting, add a MVM
instruction to the program as shown in the example below. This example
assumes the control block is an integer file.
MVM
Source N7:0
Dest 0OFF
Mask B3:0
Application Examples
1. Application example 1 is shown below. It indicates how you can
implement continuous operation of a message instruction.
2. Application example 2 begins on page 18–11 through 18–12. It involves
a SLC 5/02 processor and a SLC 5/01 processor communicating on a
DH–485 link. Interlocking is provided to verify data transfer and to shut
down both processors if communications fails.
Operation: A temperature-sensing device, connected as an input to the
SLC 5/02 processor, controls the on-off operation of a cooling fan,
connected as an output to the SLC 5/01 processor. The SLC 5/02 and
SLC 5/01 ladder programs are explained in the figure on page 18–11.
3. Application example 3 appears on page 18–13. It shows how you can use
the timeout bit TO to disable an active message instruction. In this
example, an output is energized after five unsuccessful attempts (two
seconds duration) to transmit a message.
Example 1
Operation Notes
Bit B3/1 enables the MSG instruction. When the MSG instruction done bit
is set, it unlatches the MSG enable bit so that the MSG instruction will be
enabled in the next scan. This provides continuous operation.
The MSG error bit will also unlatch the enable bit. This provides
continuous operation regardless of errors.
] [
B3
1
(U)
N7:0
15*
(EN)
(DN)
(ER)
MSG
READ/WRITE MESSAGE
Read/write WRITE
Target Device 500CPU
Control Block N7:0
Control Block Length 7
END
] [
N7:0
13*
] [
N7:0
12*
0
1
2
* MSG instruction
status bits:
12 = ER
13 = DN
15 = EN