Rockwell Automation 1785 PLC-5 User Manual download pdf (Page 109)

Programming Techniques

Chapter 7

7-12

Block Transfer Instructions

It is impossible to guarantee that both processors are executing a given BTW or

BTR at the same time, since the remote I/O scans are not synchronized, and

since the block transfer data is not transferred instantaneously by both PLC-5

processors. The BTR files in the secondary processor always reflect

information contained in the primary processor, except with a time delay.

(Refer to appendix B, “Reference Information” for more information about

block transfer instruction time delay.)

To avoid the possibility of block transfer data changing during the scan of the

program, you can buffer data at the beginning of the ladder program to ensure

continuity.

Another potential problem with block transfer instructions is that during

switchover, the secondary scanner may skip one or more remote adapters. This

may happen because the “new” primary scanner (previously the secondary) was

in a different place in its remote I/O chassis scan.

Message Instructions

In your backup system, the secondary processor is disconnected from the Data

Highway Plus link. Therefore, it cannot execute message instructions. When

programming, you must ensure that when a switchover occurs, that any

messages which were running in the primary processor are subsequently

enabled in the secondary processor. You can do this by conditioning message

instruction rungs in both the primary and secondary processors using the

primary/secondary bit of the 1785-BCM status word (Figure 7.5). If necessary,

you can pass the results of a message addressed to the primary processor over

the HSSL to the secondary processor.

Figure 7.5

Message

Instruction Rung

PRI/SEC

Message

)(

PID Control Files

When using PID instructions, you can have a 23-word integer (N) control file or

an 81 word PID (PD) control file. (PID control files are not available on the

PLC-5/15 or the PLC-5/25 processor.) You can send all or part of these control

files, based on how you back up your PID instructions and the total amount of

data to be sent by the BCM modules. Keep in mind that the control files

contain such things as your setpoint, gains, and words that are used by the PID

instruction for internal storage and should not be manipulated unnecessarily.

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Rockwell Automation 1785 PLC-5 User Manual Page 109