Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual - Manual (Page 40)

Chapter 3
VMEbus Interface
3-2
Two software-selectable bus-release modes are provided:
When set to: The PLC-5/VME processor:
ROR releases control of the VMEbus immediately after the current data-transfer
operation if it sees one of the bus-request lines asserted; otherwise it remains
“parked” on the bus.
RWD once granted the bus, keeps ownership of the bus for the duration of a series of
contiguous data transfers (e.g., a copy operation), after which it relinquishes
control of the bus (i.e., does not stay parked on the bus).
There is one exception—when set to RWD, the PLC-5/VME processor
always relinquishes the bus after the current data-transfer operation
if BCLR is asserted. Thus, when used with a priority arbiter, the
PLC-5/VME processor honors higher-priority requests even when in
the midst of a contiguous copy in RWD mode. To configure your
system for this latter case, the PLC-5/VME processor must be using
bus-request level 1 and the separate system controller must be set to
priority arbitration.
Three of the front-panel LEDs show VMEbus state information:
When this LED is lit: It means that:
SYSFAIL the PLC-5/VME processor is driving the VMEbus SYSFAIL signal.
master-access the PLC-5/VME processor is performing a VMEbus cycle.
slave-access a VMEbus master is performing an A24 slave access to the
PLC-5/VME processor.
Important: The PLC-5/VME processor does not respond to the VMEbus
SYSRESET signal if it is in a faulted state. In a faulted state, only a
power-on reset resets the processor.
Bus-Release Modes
VME LEDs