Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 94)

Selecting Interrupt Routines
Chapter 7
7-12
Fault in a Remote I/O Chassis
In general, when a remote I/O chassis faults, the processor sets an I/O rack
fault bit and then continues scanning the program and controlling the
remaining I/O. The outputs in the faulted rack remain in their last state or
they are de-energized, based on how you set the last state switch in the
1771 I/O chassis.
ATTENTION: If outputs are controlled by inputs in a different
rack and a remote I/O rack fault occurs (in the inputs rack), the
inputs are left in their last non-faulted state. The outputs may
not be properly controlled and potential personnel and machine
damage may result. Make sure that you have recovery methods.
Recovering from a ProcessorĆResident Local I/O or Remote I/O
Rack Fault
In the PLC-5 processor, you can monitor I/O rack faults using processor
status bits and then recover from the fault using a fault routine or
ladder logic.
Using Status Bits to Monitor Rack Faults
There are two types of status bits used to display information about your
I/O system: global status bits and I/O rack status bits.
The global status bits are set if a fault occurs in any one of the
logical racks.
Processor Possible Logical Rack Bits
PLCĆ5/10Ć5/12, or Ć5/15 4
PLCĆ5/25 8
Each bit represents an entire rack, no matter how many chassis make up a
rack. (Remember that you can have up to four chassis configured as
quarter racks to make up one logical rack.) These bits are stored in the
lower eight bits of words 7, 32, and 34 of the status file.
For more information on these global status bits, see your programming
software documentation set.