Commands – Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual User Manual
Page 45
Chapter 3
VMEbus Interface
3-7
WRDY is used by another VMEbus master to determine whether or not the
PLC-5/VME processor is ready to receive a command. The VME master
processor should check that WRDY is set before it writes a command
value to the Command High/Command Low registers. This prevents the
VME master processor from accidentally overwriting a previously written
command.
The Command High/Command Low registers are a 1-deep FIFO. A
WRDY bit of 1 indicates that the command register FIFO is empty and that
the VME master processor may write a command value into the command
registers. Before the write cycle is completed, the processor hardware
clears the WRDY bit to indicate that the command register FIFO is full and
so that no other commands are sent. When the processor reads the FIFO to
process the command, the FIFO is emptied and the WRDY bit is
automatically set so that the processor can send a new command.
When a single PLC-5/VME processor is controlled by two or more master
processors, the LOCK bit acts as a semaphore to prevent the processor
from accidentally overwriting another processor’s commands.
A master processor attempts to get the LOCK bit by reading the
Command/Control/Lock register. If the LOCK bit is 0, that processor has
exclusive control. This is the only processor thats sees a LOCK bit value of
0; all other processors reading the Command/Control/Lock register see a
value of 1. The master processor executes its command and then clears the
LOCK bit in the Command/Control/Lock register so that another processor
can execute its command.
Command High Register
Command Low Register
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0D
offset
0F
Commands are the primary form of communication from a separate
VMEbus CPU to the PLC-5/VME processor. A command is sent by
placing one of the following 32-bit values in the command registers.
31
24
23
000000001 Address of command block in VME A24 space
0
31
24
23
000000001
Addr of cmd blk in VME A16 space
0
16 15
Commands