The I/O modules provide the field interface for the RTU for monitoring and control of process signals. These modules can be inserted in the 4U sub-rack at any of the designated
I/O slots. Through the use of expansion racks more I/Os can be added to the RTU. A single CPU processes the entire I/O of the racks.
The main rack of the RTU consists of the processor module, which interfaces with the local rack I/O as well as the expansion rack I/O over high-speed digital links. The expansion racks are connected to the main rack over a dual-redundant bus. Each expansion rack is fitted with an I/O scanner module, which performs its local rack I/O scanning, and handles communications with the CPU over the I/O expansion bus. The I/O modules exchange I/O data, Sequence of Events (SoE), and diagnostic/configuration information of the modules/channels. All modules are synchronized with the CPU clock over the I/O bus.
For I/O, two high-speed digital links are implemented the local I/O bus, and the expansion I/O bus. The local I/O bus is based on the popular and rugged CAN bus. It has been widely used in industrial control and automotive industry. CAN bus provides high immunity to external noises present in an industrial environment, and has built-in error handling and fault confinement features. The expansion I/O bus is based on the industry-standard ARCNet protocol. ARCNet is a deterministic, reliable bus with proven track record. The expansion bus can operate at a maximum speed of 10Mbps. Using the time-deterministic nature of the ARCNet protocol, all I/O is scanned by the CPU within a fixed time, called the RTU I/O scan cycle.
All I/O modules are designed with an onboard dedicated 32-bit microcontroller. This aids in faster I/O scanning and in maintaining of time-stamps as close to the field interface as possible. Further, I/O module specific intelligence is offloaded by the CPU to these microcontrollers.
While interfacing with field signals, all I/O modules provide a minimum of 1kV isolation between the field and the internal logic. Further, all field signals that enter the I/O modules are protected against transients. Module-specific input protection, if any, is also provided to prevent damage to the module in case of inadvertent inputs or wrong wiring. Hot-swap feature of the I/O modules allows field maintenance and troubleshooting without affecting overall system downtime.