modified, or serviced without affecting any other drop (except from loss of data
from the drop being serviced). Modularity also allows easy expansion from a
small system to a large system and reduces spare inventory.
Input/Output Controller . Input/output controllers consist of intelligent modules,
each performing a dedicated function. Modules are chosen to meet the specific
function including input/output mix, redundancy, geographic distribution, and
process requirements. Capability should be provided in the controller to perform
logic functions that will eliminate the need for separate programmable logic
controllers (PLCs). On critical processes, the use of backup power supplies,
redundant controller, and input/output cards should be considered. Also, these
controllers should be located as near as possible to the process to reduce the
cost of cable installation.
Communications . Communication networks in the system provide high-speed,
secure controller information exchange, as well as an open network for standard
computing hardware access where appropriate. Communications between
individual modules should be via local, independent buses that allow complete
integration of the family of modules. Open system capabilities may also allow
third-party software to be an integral part of the system.
Data Highways. For critical applications, redundant data highways should be
considered. Failure of one data highway should be transparent to the system
and should not affect system performance. An alarm message that would
activate upon failure of a redundant highway should be considered at the
operations station. Fiber optics may be useful in this application for its immunity
to noise and electrical fault isolation. Wide band width, high reliability, and noise
resistance should make fiber optic networks an attractive component of many
complex control systems. Redundant computer controls and uninterruptible
power sources should make these new technology control systems much more
accurate and reliable than traditional analog systems.