Rockwell Automation 1785-Vx0B, D17856.5.9 PLC-5 VME VMEbus Programmable Controllers User Manual - Manual (Page 73)

Chapter 6
PLC-5/VME Processor
Communications Commands
6-4
Table 6.A describes the bytes that compose the headers of command and
reply packets. We do not repeat their descriptions in the description of
each command that follows.
Important: All numbers are decimal except where noted by an “H”
for Hexadecimal.
Table 6.A
Command and Reply Packets
Header Bytes Function Description
CMD Command CMD and FNC bytes together define the command to be
executed. Command codes are included in command
descriptions later in this chapter.
STS Status If the PLC-5/VME processor detects an error, it reports error
codes in the reply packet. Zero means no error. Error codes are
described for each command, below.
Set to zero in the command packet.
STS and EXT STS (extended status) are returned in the reply
packet in response to some commands. STS bits 07-00 contain
the value F0H when reporting extended status. Status and
extended status codes that could be returned in the reply packet
are described for each command, below.
TNS Transaction
code (two
bytes) The host CPU’s driver program should generate a unique 16-bit
number for each transaction so that it can match replies to
corresponding commands. There should not be more than one
active packet with the same transaction number from any source.
Whenever the PLC-5/VME processor receives a command, it
copies the TNS value of the command packet into the same field
of the corresponding reply packet without changing the TNS
value.
FNC Function
code For a command packet, it combines with the CMD byte to define
the command. See CMD, above.
EXT STS Extended
status code If the PLC-5/VME processor detects an error, it reports extended
status codes in the reply packets of some commands. See
STS, above.
The reply packet also contains the CMD byte. The PLC-5/VME processor
copies the CMD value from the command packet into the corresponding
reply packet.
Bit Description
07 Always zero
06 Designates command or response. The host CPU resets this bit when sending a
command. The PLC-5/VME processor sets this bit to 1 when sending a reply. (0 =
command, 1 = reply)
05, 04 Not used (set to zero)
03-00 Command codes (in Hex)
Use command codes with function codes FNC to specify the type of command.
Header Bit/Byte
Descriptions