GE 90-30/20/Micro manual Programmer Communications Window, Output Scan

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Output Scan

Outputs are scanned during the output scan portion of the sweep, immediately following the logic solution. Outputs are updated using data from %Q (for discrete outputs) and %AQ (for analog outputs) memory, as appropriate. If the Genius Communications Module is configured to transmit global data, then data from %G memory is sent to the GCM, GCM+, or GBC. The Series 90-20 and Micro output scans include discrete outputs only.

During the output scan, all Model 30 output modules are scanned in ascending reference address order.

If the CPU is in the STOP mode and the CPU is configured to not scan I/O during STOP mode, the output scan is skipped. The output scan is completed when all output data has been sent to all Model 30 output modules.

Logic Program Checksum Calculation

A checksum calculation is performed on the user program at the end of every sweep. Since it would take too long to calculate the checksum of the entire program, you can specify the number of words from 0 to 32 to be checksumed on the CPU detail screen.

If the calculated checksum does not match the reference checksum, the program checksum failure exception flag is raised. This causes a fault entry to be inserted into the PLC fault table and the PLC mode to be changed to STOP. If the checksum calculation fails, the programmer communications window is not affected. The default number of words to be checksummed is 8.

Programmer Communications Window

This part of the sweep is dedicated to communicating with the programmer. If there is a programmer attached, the CPU executes the programmer communications window. The programmer communications window will not execute if there is no programmer attached and no board to be configured in the system. Only one board is configured each sweep.

Support is provided for the Hand-Held Programmer and for other programmers that can connect to the serial port and use the Series Ninety Protocol (SNP) protocol. Support is also provided for programmer communications with intelligent option modules.

In the default limited window mode, the CPU performs one operation for the programmer each sweep, that is, it honors one service request or response to one key press. If the programmer makes a request that requires more milliseconds to process than the limit for the communications window (see Note), the request processing is spread out over several sweeps so that no sweep is impacted by more than the limit (see Note).

Note

The time limit for the communications window is as follows: ∙15 milliseconds for the 311 and 321 PLCs

∙8 milliseconds for the 313, 323 and 331 PLCs ∙10 milliseconds for the 340 and 341 PLCs ∙6 milliseconds for the 350 and higher PLCs

GFK-0467K Chapter 2 System Operation 2-9