Rockwell Automation 1775-S5_SR5,D17756.5.5 User Manual PLC-3 FAMILY I/0 - Manual (Page 74)

DH and DH+ Communication
Chapter 4
4-14
When using a scanner for DH or DH+ communication, you can set up two
types of backup configurations:
two scanners in the same processor chassis for backing up the link
Both scanners are active, and each one is an independent station on the
link. You assign unique station numbers to each scanner. If you want to
send the same message through both scanners, you must program
separate message instructions.
scanners in a backup system
One scanner is in the primary system and one scanner is in the backup
system.
For detailed information on backup systems, refer to the PLC-3 Family
Controller Backup Concepts Manual (publication 1775-6.3.1).
By inserting multiple scanners in the same PLC-3 or PLC-3/10 system,
you can operate a single system on more than one link. In this
configuration, each scanner connects to a different link, and each has a
unique station on its associated link. However, all the scanners in the same
system can have either the same or different station numbers.
ATTENTION: If such a PLC-3 station is communicating
through a PLC-2 buffer file on a DH link, and all of the station’s
scanners have the same station number, then all of the scanners
transfer data through the same buffer file. This can cause
unpredictable results if several scanners try to read or write to
the buffer file at the same time (see “Accessing a PLC-3
Processor from a PLC-2 Processor,” page 4-11).
When such a PLC-3 station transmits a command message to a remote DH
or DH+ station, the thumbwheel number specified in the message
instruction determines which scanner actually transmits the command.
Operating Backup
Configurations on a DH or
DH+ Link
Operating Multiple Links in
One System