Allen-Bradley MicroLogix 1200 Reference Manual page 327

Controller
MicroLogix 1200
MicroLogix 1500 with 1764-LSP Processor 1
MicroLogix 1500 with 1764-LRP Processor 1
Communication Status Bits
The following communication status bits allow you to customize or
monitor communications servicing. See General Channel Status Block on
page 3-14 for additional status information.
Table 21.5 Communication Status Bits
Address Description
CS0:4/0 ICP - Incoming Command Pending
CS0:4/1 MRP - Incoming Message Reply Pending
CS0:4/2 MCP - Outgoing Message Command Pending
CS0:4/4 CAB - Communications Active Bit
Application Example
The SVC instruction is used when you want to execute a communication
function, such as transmitting a message, prior to the normal service
communication portion of the operating scan.
CS0:4
0000
MCP
You can place this rung after a message write instruction. CS0:4/MCP is set
when the message instruction is enabled and put in the communications
queue. When CS0:4/MCP is set (1), the SVC instruction is evaluated as true
and the program scan is interrupted to execute the service
communication's portion of the operating scan. The scan then resumes at
the instruction following the SVC instruction.
The example rung shows a conditional SVC, which is processed only
when an outgoing message is in the communications queue.
You may program the SVC instruction unconditionally
NOTE
across the rungs. This is the normal programming
technique for the SVC instruction.
Communications Instructions
Channel Select Setting Channel(s) Serviced
1 0
0
0
2 1
3 both 0 and 1
SVC SVC
Service Communications
Channel Select
Publication 1762-RM001C-EN-P
21-27
0001h