GFK-0356Q
Chapter 3 Baseplates
3-11
3
Differences Between Remote and Expansion Racks
Basically, Remote racks provide the same functionality as Expansion racks, but with the longer
distance (700 feet/213 meters verses 50 feet/15 meters for Expansion racks) capability. To
minimize unbalanced ground conditions, Remote baseplates have extra isolation circuitry.
Unbalanced ground conditions can occur when systems are located long distances from each other
and do not share the same ground system. However, distance is not always the problem; even
racks that are mounted near each other can experience problems if the system is not grounded
properly. See Chapter 2 for grounding information.
The use of Remote racks requires a special consideration pertaining to scan time. In order to
operate at long distances, the I/O Bus runs at a lower clock speed (compared to that used for
Expansion racks) when communicating with Remote racks, which will have an impact on
performance. The impact will be relatively small for discrete I/O and slightly more for other
modules, such as the High Speed Counter or Genius Communications Module. The increase in
time needed to communicate with modules in a remote baseplate will usually be small with respect
to the overall scan time. For more detailed information on scan time calculations, refer to Chapter
2 of GFK-0467, the
Series 90-30/20/Micro PLC CPU Instruction Set Reference Manual
.
Another important scan time consideration is the cable type used for communicating at longer
distances. Data propagation delay must be minimized to ensure proper system timing and margins.
Any deviation in cable type may result in erratic or improper system operation. Suggested cable
types are specified in the “Cables” chapter in the IC693CBL300/etc. data sheet.
Mixing Expansion and Remote Baseplates in a System
Expansion and remote baseplates can be used in the same system as long as certain requirements
are met:
Ŷ
You do not exceed the 50 foot (15 meter) maximum cable distance from the CPU to the last
Expansion baseplate
Ŷ
You do not exceed the 700 foot (213 meter) maximum cable distance from the CPU to the last
Remote baseplate.
Ŷ
The cable type recommended for use with Remote baseplates must be used throughout the
system. The exception to this requirement is that the prewired 3 foot (1 meter) cable,
IC693CBL300, can be used as a Wye adapter to simplify the custom cable assembly associated
with the “daisy chain” connections between baseplates. Information on building cables for use
with Remote baseplates can be found in the “Cables” chapter in the IC693CBL300/etc. data
sheet..
