Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 92)

Publication 1785-UM012D-EN-P - July 2005
6-20 Communicating with Remote I/O
• When performing block-transfers (controller-resident local or remote
I/O) in any PLC-5 controller, clear the output image table
corresponding to the block-transfer module rack location before
changing to run mode. If you do not clear the output image table, then
you encounter block-transfer errors because unsolicited block-transfers
are being sent to the block-transfer module (i.e., if a block-transfer
module is installed in rack 2, group 4, clear output word O:024 to 0; do
not use the word for storing data).
• If you use remote block-transfer instructions and have the timeout bit
(.TO) set to 1, then the controller disables the 4-second timer and
continues to request the block-transfer for 0-1 seconds before setting
the error (.ER) bit.
• A PLC-5 controller with at least one channel configured as an adapter
could incur a non-recoverable fault when you switch it from run to
program mode.
To avoid this possibility, program the scanner to request only two or three
block-transfers from the PLC-5 adapter at one time by conditioning the
block-transfer instructions with the done/error bits.
For Controller-Resident Local Racks
The following are programming considerations when you are
block-transferring data in a controller-resident local rack.
• Within the controller-resident local rack, limit the number of
continuous-read block-transfers to 16 transfers of 4 words each or 8
transfers of 64 words each. If you attempt to exceed this block-transfer
limit, a checksum error (error code -5) occurs.
• Block-transfer instructions to any of the following modules residing in
the controller-resident local rack result in frequent checksum errors.
– 1771-OFE1, -OFE2, and -OFE3 modules; all versions prior to series
B, revision B
– 2803-VIM module, all versions prior to series B, revision A
– IMC-120, all versions
• To eliminate the checksum errors, replace your modules with the current
series and revision. If replacement is not possible:
1. Go to the controller configuration screen in your programming
software.
2. With the controller in program mode, set user control bit 4 (S:26/4) to 1
(the local block-transfer compatibility bit).
3. Change the controller mode from program to run.