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Modbus Plus for DIO Servicing
26
870 USE 103 00 May 2001
Strategies for Distributed I/O Servicing
Overview Modbus Plus networks can be used to service multi-purpose control applications, or
they can be organized for the most efficient servicing of distributed I/O devices. This
section compares the two approaches.
Network
Function
In multi-purpose control applications, the network is designed to allow
communication between in programmable controllers, operator interfaces, and other
kinds of devices.
For efficient servicing of distributed I/O, the network is designed to allow
communication between one programmable controller and a group of I/O modules.
Message Timing In multi-purpose control applications, timing can vary according to the current
processing requirements of each node’s internal program.
In efficient distributed I/O servicing applications, the timing of message transactions
must be predictable to allow deterministic timing of the I/O control process.
Size In general applications, up to five networks can be joined by Bridge Plus devices to
extend the cable length to 2250 m (7500ft) and the node count to 320 nodes.
In distributed I/O applications, messages are transacted on the local network only.
BridgePlus devices are not applicable to networks used for distributed I/O.
Recommen-
dation
Multi-purpose networks are not recommended for servicing I/O control applications
in which I/O timing must be deterministic.