User Manual

48 • I/O Modules
Special Modules
WAGO-I/O-SYSTEM 750
I/O Modules
2.1.1.8.2.3 Access Procedure
Unmasking the mailbox by setting bit 5 in the control byte is required for exe-
cuting mailbox commands. The module confirms this by setting bit 5 in the
status byte. In order to execute a mailbox command, query parameters and the
opcode of the command must be written in the output process image. Since a
change in the opcode and/or the toggle bit is a trigger for the processing of a
command, the query parameters must be written into the output process image
either at the same time or previously.
The module confirms the processing of the command by inserting a response
telegram in the mailbox area of the internal data bus input data. The response
evaluation must occur at the same point at which the opcode and toggle bit are
identical with the query contents; i.e., these are mirrored. The processing time
in the module may require several bus cycles. Some special commands trigger
a longer process (e.g., search for devices within range). For these commands,
the module's response confirms that the process has begun. The results of
longer lasting processes can be queried after completion by other commands.
The toggle bit is necessary for executing two mailbox commands with the
same opcode (but possibly differing parameters) one after the other.
Note
The use of mailbox commands implements a confirmed service. The module
provides information via return value on the successful execution of the
command or errors that occur. If errors occur, it may be that not all response
bytes contain valid data.
The following diagram (see Figure 17) describes the request and processing of
a mailbox command. The process data are displayed as follow in this case:
[ parameter 0-x | toggle | opcode | internal byte | control/status byte ]
Initially, any process data may be present in the output and input process im-
age. After entering the opcode and/or toggle bit, as well as switching the mail-
box on using bit 2
5
in the control byte, the mailbox command is transmitted
and a query is started. In the input process image, the query is received, proc-
essed and confirmed with bit 2
5
. This confirmation and the new process data
are sent to the output process image. Here, the data is evaluated. The next
command can be transmitted.