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Modbus Plus for DIO Servicing

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.

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Momentum Modbus Plus 170 PNT Series Betriebsanweisung Seite 26