Multiple CPU Modules

The standard RTU contains a single CPU module that controls all activities in the RTU. Some critical sites
may require a spare CPU module that is seamlessly activated should a failure be detected. The MOSCAD RTU
provides a Dual CPU module mode of operation: the normal CPU module is installed in the left-most position
in a standard motherboard and a second CPU module installed in the immediately adjacent position; eachmodule
is downloaded with essentially identical application programs. In the normal state the second CPU module is
off-line (connected to the I/O modules but not controlling them). The active CPU module periodically sends an
I’m OK message to the secondary CPU module; if the secondary CPU module ever fails to receive the I’m OK
message within the allotted time, it will assume control of the I/O modules and begin all communication tasks.
The RTUmay, following such a switch, send an advisory message to the SCADA Manager or elsewhere to notify
Maintenance that a service call is required.
There are other times when multiple CPU modules are required within a single RTU. The number of required
RS-232 connections may exceed what a single CPU module may support, the amount of data through the several
RS-232 connections may place an excessive time-burden upon a single CPU module, or multiple radios and/or
wirelines may need to be connected to the RTU. Multiple CPU modules are clearly a solution: the CPUs may be
interconnected via the RS-485 2-wire multidrop ports and exchange data using the MDLC protocol’s store-&-
forward capability. The rackmount configuration is the recommended approach: special motherboards are
available that allow multipleCPUmodules with oneCPUcollecting data from associated I/O modules and sharing
that data with the other CPU modules. Refer to the rackmount discussion in the Physical Configuration section
for more details.
The RS-232 Multiplexer (Mux) is available to expand a single full RS-232 port on the CPU module to four
ports. The Mux obtains operating power from the power supply/battery and may be installed within the standard
NEMA configuration or in the rackmount configuration but not in the small NEMA configuration (no space for
the Mux). The Mux operates under the control of Port 2 (or Port 3 when an Async interface is present) on the
CPU module and may be set for broadcast CPU-to-device with first-come-first-served response or set for directed
CPU-to-device input/output; the Mux may also be set to echo characters received from some device back to the
device. All connected devices must support the RTS/CTS/DTR mode of operation.