CTI 2572-B / 2500C-2572-B Installation and Operation Manual 65
You might enter a V memory address of a valid command block, but it is not the command block you want to
process. In this case, you will probably not get an error, just the wrong result.
Command Control Errors
One common error in manipulating the Command Control bits is a failure to assert ERR ACK (Error
Acknowledge) after the module has raised the CMD ERR (Command Error bit). If you fail to acknowledge the
error, the associated command slot will appear to "lock up" on the command slot. In actuality, the module
software is waiting on the PLC acknowledgment before proceeding. You can tell that the module is not really
locked up by observing the timer value in the lower byte of module WX1 increment and that you have a
Command Error by examining the applicable error bit. In addition, if you have triggered other commands,
they will continue to operate (unless an error also occurs on these command slots).
Another potential error is failing to observe the timing protocol for coupled mode. You must wait for the
module to assert CMD Busy and then lower CMD Trigger. Failure to observe the timing may cause the
command slot to appear to "lock up". Actually the module is waiting to see the PLC lower the CMD trigger.
See APPENDIX B. PLC COMMAND INTERFACE for timing details.
Errors in manipulating the Command Control bits may cause "multiple triggering," resulting in an error code.
Certain commands, such as those to create a connection, must be run only once. Improperly constructed PLC
logic may repeatedly trigger the command, resulting in an error code such as 0x00A7 (duplicate connection).
10.7. Development and Debugging Tips
Manual Triggering
You can test your command blocks independently from the PLC logic that actually executes them by
manipulating the command control bits manually. First, place the PLC in program mode so that the PLC logic
will not be executing. Then you can manipulate the bits as shown below. If you are not familiar with the 2572
PLC Command Interface, you should refer to APPENDIX B. PLC COMMAND INTERFACE.
The command control bits are located in the 4th module word. For example, if you logged the module in
starting at Word 1, then the command control bits will be located in WY4. See the WX/WY Quick Reference in
Appendix C of this manual. There are 4 sets of control bits, one set for each command slot. Any command slot
can be used in any order.
Command Control
Bits - Slot 1
Command Control
Bits - Slot 2
Command Control
Bits - Slot 3
Command Control
Bits - Slot 4
Within each set of four Command Control bits:
• The first bit is the Error Acknowledge,
• The second bit is the Command Mode bit,
• The third bit is the Command Trigger,
• The fourth bit Abort Trigger.
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