1766-RM001A-EN-P MicroLogix 1400 Programmable Controllers ...

396 Communications InstructionsSVC - ServiceCommunicationsSVCService CommunicationsChannel Select 1Instruction Type: outputExecution Time for the SVC InstructionController When Rung Is: (1)MicroLogix 1400True39.8260 µs (CH0)5.9042 µs (CH1)36.5800 µs (CH0 & CH1)False0.1933 µs (CH0)0.1857 µs (CH1)0.1774 µs (CH0 & CH1)(1) This value for the SVC instruction is for when the communications servicing function is accessinga data file. The time increases when accessing a function file.Under normal operation the controller processes communications onceevery time it scans the control program. If you require thecommunications port to be scanned more often, or if the ladder scan islong, you can add an SVC (Service Communications) instruction to yourcontrol program. The SVC instruction is used to improve communicationsperformance/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 communicationsthat need to take place. You can place the SVC instruction on a rungwithout any preceding logic, or you can condition the rung with anumber of communications status bits. The table on page 397 shows theavailable status file bits.TIPThe amount of communications servicing performed is controlled by theCommunication Servicing Selection Bits (CSS) and Message ServicingSelection Bits (MSS) in the Channel Communication Configuration File.Refer to Communication Servicing Selection and Message ServicingSelection on page 410 for more information.For best results, place the SVC instruction in the middle of the controlprogram. You may not place an SVC instruction in a Fault, DII, STI, or I/OEvent subroutine.Channel SelectWhen using the SVC instruction, you must select the channel to beserviced. The channel select variable is a one-word bit pattern thatdetermines which channel is serviced. Each bit corresponds to a specificchannel. For example, bit 0 equals channel 0. When any bit is set (1), thecorresponding channel is serviced.Publication 1766-RM001A-EN-P - October 2008