GE GFK-1541B manual Exchange Limitations and Recommendations, Maximum Number of Exchanges

5

Configuration Planning

Producer and Consumer Periods for PLCs

Follow these guidelines for the producer and consumer periods.

ƒSet the producer period and the consumer period to the same value. This makes the system easier to troubleshoot and makes network resource usage more efficient.

ƒDo not produce and consume data faster than is required by your application. This reduces the load on the network and on the devices, providing capacity for other transfers. For example, an exchange intended for an HMI host usually does not need to be produced any faster than 500 milliseconds or so. In fact, a 1 to 3 second producer period may be sufficient for this case.

Exchange Limitations and Recommendations

The limitations and recommendations for consumer and producer exchanges are as follows:

Maximum Number of Exchanges

The maximum number of exchanges can be divided up into any combination of producer and consumer exchanges.

ƒFor a single Series 90-70 PLC system, which may contain multiple Ethernet Interfaces, the maximum combined number of exchanges that can be configured is 255.

ƒFor the Series 90-30 CPU364 and the Series 90-30 CPU374, the maximum number of exchanges that can be configured is 128.

Maximum Data Size of an Exchange

The total data size of a single exchange cannot exceed 1400 bytes. The total size is defined to be the sum of the data lengths of all of the variables within the list.

Number of Variables

ƒUp to 100 variables can be configured per exchange.

ƒThe minimum is one variable per exchange.

ƒDifferent exchanges may have different variables, and they may share some or all of the same variables even if the exchanges are produced and/or consumed at different rates.

It is possible to configure more EGD than a PLC can transfer either due to processing limitations in the Ethernet Interface or in the underlying network (especially in 10Mbit networks). If you configure too much EGD, you will experience high levels of consumer timeouts in some or all of your consumed exchanges. In this case, you must reduce the EGD load. Possible approaches to reducing the load include:

ƒIncrease the production period (especially if the period is more frequent than 2x the minimum time in which the data is needed).

ƒRedefine the exchanges to use fewer exchanges, each with more data. This can often be achieved by making use of EGD groups (rather than producing a directed exchange to several destinations, a single exchange produced to the group can contain all the data and each consumer can transfer the data of interest from the exchange).

ƒFor Series 90-70 PLCs, consider adding an addition Ethernet Interface to the rack and spreading the EGD exchanges.

5-6 TCP/IP Ethernet Communications for the Series 90™ PLC User's Manual– May 2002 GFK-1541B