Siemens MOBY FC 56 Technical Description page 82

Entry/Exit Check of an MDS in an SLG Field
6.1 Definition of Terms
Each SLG that processes an MDS writes its station-specific SLG number
(SLG_number) to the system area of the MDS. The SLG number is written
automatically before the first command to be executed on the MDS. The SLG
number can be used to detect whether it is a new or an old data carrier that is
currently passing the SLG station.
SLG number
The SLG number is parameterized with the SLG_number parameter in UDT 10
(see chapter 3.1). Be sure to configure a different SLG number for every SLG in a
system.
New MDS
SLG numbers (SLG_number on MDS and station-specific SLG_number) do not
correspond
Old MDS
SLG numbers correspond
Current MDS
All active commands apply to this MDS
Next MDS bit
Internal status flag in the filehandler
The old MDS remains current (all commands relate to this MDS) until it is closed
by a NEXT command. All subsequent commands on this SLG refer to the next
new MDS. This is achieved by having the filehandler set the "next MDS bit" when it
receives a NEXT command. If the old MDS reenters the SLG transmission window
as the next MDS (SLG numbers correspond), no command is issued (the "next
MDS bit" remains set). When a new MDS enters the transmission window, the
"next MDS bit" will be reset and the station-specific SLG number will be written to
the system area of the MDS (the new MDS thus becomes the old/current MDS).
This mechanism is based on the assumption that all SLG stations in the
system have different SLG numbers (set by the "SLG_number" parameter;
see section 3.1).
6-2
FC 56
J31069-D0155-U001-A0-7618