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CTI 2572 Installation and Operation Guide
131
Command Blocks
Command Blocks contain information which specifies how the command is to be executed. The
standard Command Block consists of 16 contiguous words located in the first 64k of PLC V memory
and has the following general structure.
Offset Contents
0 Error Word
1 Command Code
2 Connection Number
3 Command Parameter 1
4 - 15 Additional parameters
Error
Word
Assuming the Command Block is valid, the module will write an error code into this
location when it encounters an error in executing the command. The error code can be
used during development as an aid to de-bugging logic. It may also be used by the
PLC logic as an indication to execute specific error processing logic.
Command
Code
The Command Code is a number which uniquely defines the command you wish to
execute.
Connection
Number
The connection number identifies the instance of the protocol manager with which
you are communicating. A connection number is similar to a file handle used in other
operating environments. The connection number is established when a C
REATE
CONNECTION or CREATE SOCKET command is executed. During this process, a
specific protocol manager is loaded and associated with a particular physical port or a
network socket. If the connection is created using PLC logic, the programmer can
assign a connection number. If the connection is automatically created by the module,
the module will assign a connection number.
The high byte of the Connection number should always contains hex 4B. The module
uses this byte as a Command Block "signature". The use of the signature byte
provides a way for the module to verify that the address you loaded in the command
slot indeed points to a valid command block and not some arbitrary area of V
memory. When the module detects that you have referenced an area of V memory
where the high byte of offset 3 does not contain hex 4B, it asserts the CMD Err and
PLC Err. Since the module assumes that a block without a signature byte is not a
command block but a reference error, it does not attempt to process the block of V
memory and does not write any error information back into offset 0 of the block.
Command
Parameters
The command parameters provide command execution information. For example, for
a Read Command they may specify where to obtain the data, how many words to
read, and where to store the results. The number of command parameters required
will vary with different commands.