Siemens MOBY FC 56 Technical Description page 83

6.2 Entry/Exit Checking Mode 0 (MDS_IO_control 0)
After receiving a command from the FC 56, the filehandler continuously attempts
to establish communication with the MDS via the connected SLG. As soon as an
MDS appears in the transmission window, the filehandler begins to process the
command. With this method, the user can issue a command to the filehandler at
any time. This command is stored until it can be processed on a valid MDS.
The NEXT command plays a decisive role in this type of control. The issuing of the
NEXT command means that:
S the processing of the old/current MDS is terminated
S immediately after NEXT, a command can be issued that will not be processed
until the arrival of a "new" MDS.
S the filehandler ensures that a command will only be executed on the next "new"
MDS. Thus, if the old (already completely processed) MDS returns as the new
MDS into the SLG transmission window, the command that was issued will not
be executed.
Issuing the NEXT command is always permissible if:
S the old/current MDS is still in the SLG transmission window
the old/current MDS has already left (no MDS in SLG transmission window)
S new/next MDS is in the SLG transmission window (the new MDS thus becomes
the current one)
MDS_IO_control 0 allows a continuous monitoring of the MDS flow control. If a
new MDS enters the SLG transmission window before the old/current MDS is
terminated by a NEXT command, the following occurs:
1. If a command has been issued (not NEXT), then an error message is output.
The user has not terminated the old/current MDS with NEXT or an MDS for
which this command is not intended has entered the transmission window.
2. If the NEXT command has been issued, the old/current MDS will be terminated
and the new MDS, which is now in the SLG transmission window, becomes the
current MDS (and thus also the old MDS). All further commands will be
executed on this current MDS until a further NEXT terminates the processing.
3. An error situation arises if the new MDS leaves the SLG transmission window
again, since no actions were performed on the new MDS while it was in the
window. The error message will not appear until a further command is issued.
FC 56
J31069-D0155-U001-A0-7618
Entry/Exit Check of an MDS in an SLG Field
6-3