Rockwell Automation 1747-DSN DISTRIBUTED I/O SCANNER - Manual (Page 35)

Chapter 4
Operation and Troubleshooting
4–5
I/O Block Hold Last State Operation
During normal operation, the I/O block outputs reflect the SLC processor
output image. However, there are several conditions under which the I/O
block outputs will not reflect the SLC processor output image, but will either
be reset to 0 or will hold their last state. In these cases, the state of the I/O
block outputs will be determined by the output control information sent by
the scanner and the Hold Last State switch on the I/O blocks.
The I/O block outputs will be reset to 0 regardless of the I/O Block Hold Last
State switch setting or the SLC processor output image whenever :
•
the scanner loses power
•
the SLC processor is taken out of the run mode
The I/O block will use its Hold Last State switch to determine the state of its
outputs whenever :
•
the Output Disable Bit in the Output Control Word is set to 1 and the SLC
processor is in run mode
•
the I/O blocks and scanner lose communication (e.g. broken cable)
For details on I/O block Hold Last State operation, consult the I/O block
User Manual.
!
WARNING:
Use of the Hold Last State switch can result in outputs remaining
energized when not under the control of the SLC processor. We
recommend the use of this function only by experienced SLC
programmers.