Publication 1766-RM001A-EN-P - October 2008
396
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 397 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
Execution Time for the SVC Instruction
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 1400
39.8260 µs (CH0)
5.9042 µs (CH1)
36.5800 µs (CH0 & CH1)
0.1933 µs (CH0)
0.1857 µs (CH1)
0.1774 µs (CH0 & CH1)
TIP
The amount of communications servicing performed is controlled by the
Communication Servicing Selection Bits (CSS) and Message Servicing
Selection Bits (MSS) in the Channel Communication Configuration File.
Refer to Communication Servicing Selection and Message Servicing
Selection on page 410 for more information.
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