Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual - Manual (Page 71)

Chapter 6
PLC-5/VME Processor
Communications Commands
6-2
A PCCC reply packet has the following format:
7 6 5 4 3 2 1 0
Byte
0
1
2
3
4
5
6
7
8
9
0 1 0 0 COMMAND
TNS – first byte
TNS – second byte
OPTIONAL DATA
(up to 243 bytes)
LNH - first byte
LNH - second byte
10 OPTIONAL EXTENDED STATUS (EXT STS)
REMOTE ERROR 0
Reserved
Reserved
Reserved
Bit
Reserved
11
11/12
(DST)
(PSN)
(SRC)
(PSN)
Command Description
LNH Length of the optional portion of the reply packet in bytes. The first byte
of LNH is the high-order byte (actual length = LNH – 4)
. 1
COMMAND Copied from the associated command packet.
REMOTE ERROR If nonzero, the PLC-5/VME processor has encountered a problem
attempting to process the command. 0001-1110 represent error codes
listed separately. 1111 indicates that the EXT STS field contains an error
code.
TNS Copied from the associated command packet.
OPTIONAL
EXTENDED STATUS This field contains an error code when the REMOTE-ERROR field has
the value 1111.
OPTIONAL DATA This contains data returned as part of the reply. The value(s) and size of
this field are specific to the type of command. Whether this field starts at
offset 10 or offset 11 depends on whether the specified command is
defined to return the extended status byte.
1 As we stated early in this chapter, all numerical data in the extended PCCCs is defined in Intel format,
however, this is the exception. This is in the Motorola format.
The host CPU driver program is responsible for leaving sufficient space for
the reply packet immediately after the command packet in memory. The
actual size of the reply packet depends on the specific type of
PCCC command.