Allen-Bradley MicroLogix 1200 Reference Manual

Publication 1762-RM001C-EN-P
21-26 Communications Instructions
SVC - Service
Communications
Instruction Type: output
Under normal operation the controller processes communications once
every time it scans the control program. If you require the
communications port to be scanned more often, or if the ladder scan is
long, you can add an SVC (Service Communications) instruction to your
control program. The SVC instruction is used to improve communications
performance/throughput, but also causes the ladder scan to be longer.
Simply place the SVC instruction on a rung within the control program.
When the rung is scanned, the controller services any communications
that need to take place. You can place the SVC instruction on a rung
without any preceding logic, or you can condition the rung with a
number of communications status bits. The table on page 21-27 shows the
available status file bits.
For best results, place the SVC instruction in the middle of the control
program. You may not place an SVC instruction in a Fault, DII, STI, or I/O
Event subroutine.
Channel Select
When using the SVC instruction, you must select the channel to be
serviced. The channel select variable is a one-word bit pattern that
determines which channel is serviced. Each bit corresponds to a specific
channel. For example, bit 0 equals channel 0. When any bit is set (1), the
corresponding channel is serviced.
SVC
Service Communications
Channel Select 1
SVC
Table 21.4 Execution Time for the EQU and NEQ Instructions
Controller
When Rung Is:
(1)
(1) This value for the SVC instruction is for when the communications servicing function is
accessing a data file. The time increases when accessing a function file.
True False
MicroLogix 1200 208
µ
s + 1.6
µ
s per word 0.0
µ
s
MicroLogix 1500 1764-LSP or
1764-LRP with one channel selected
166
µ
s + 1.4
µ
s per word 0.0
µ
s
MicroLogix 1500 1764-LRP Processor
with both channels selected
327
µ
s + 1.4
µ
s per word 0.0
µ
s
NOTE
The amount of communications servicing performed is
controlled by the Communication Servicing Selection Bit
(CSS) and Message Servicing Selection Bit (MSS) in the
Channel 0 Communication Configuration File.

Allen-Bradley MicroLogix 1200 Specifications

General

Brand	Allen-Bradley
Model	MicroLogix 1200
Category	Control Unit
Language	English