Rockwell Automation 1785-BCM_BEM, D17856.5.4 PLC-5 Backup Communication Module User Manual - Manual (Page 76)

Operating Your PLC-5 Backup System
Chapter 5
5-12
Each byte of word W2 is divided into two groups of four bits corresponding to
the module’s channels (Figure 5.7). The three least-significant bits of each
group corresponds to three switches of switch assembly SW2 which is used to
specify the configuration of module channels. The most-significant bit
corresponds to the error bit for the respective channel.
Refer to Chapter 8, Diagnosing Faults, for further information about these bits
of the Auxiliary Diagnostics/Status Words.
Secondary Processor Remote Programming
Important: The programming device must be connected through the Data
Highway Plus network and must not bypass the relay in the 1785-BCM
module. If you were to connect the device directly to a processor, it must be
connected to the primary processor. Therefore, if your system were to switch to
the secondary system, you must then move your device to your new primary
processor.
In the PLC-5 backup system, only the primary processor is connected to the
link. The secondary processor is not physically connected, but it communicates
with the secondary 1785-BCM module making this processor think that it is on
the Data Highway Plus network. This separate link prevents Data Highway
faults in the secondary processor.
The 1785-BCM series B backup module provides remote programming
capability for your secondary processor. This means that even with the
programming device directly connected to the primary processor, the secondary
PLC-5 processor memory can be programmed and/or monitored.
Important: If you are communicating with the primary processor via a
programming terminal, do not place it in Remote Program mode. If you do, a
switchover will occur and the new primary can go into Remote Program mode,
shutting the system down.
Important: As a guideline, you should connect over DH+ to only one
programming terminal at a time on the secondary processor.
The primary 1785-BCM module provides an access point for a programming
device to access the secondary processor. Using the 1785-BCM module, you
can communicate from a programming device across the backup modules to the
secondary processor. You must assign both the primary and the secondary
processor the same node address (using either dipswitches or Channel Config
screen in 6200 software) in the Data Highway Plus link. The 1785-BCM
module reserves the subsequent node address as an access-point address to the
secondary processor, that is, this address is accessed by a programming device
in order to program the secondary processor.