GE GFK-1541B manual Allowable Data Types in Exchanges, Producer and Consumer Period Ranges

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Allowable Data Types in Exchanges

Boolean type variables are not allowed; neither are Boolean-Array variables. If discrete point references such as %I or %Q are used to define a variable, you must define them as Byte-Array, Word-Array, or Dword-Array variables. This implies that a variable consisting of discrete point references must be defined in blocks of 8 points if it is defined as a Byte-Array, 16 points if Word-Array, and 32 points if DWord-Array. Discrete memory must be byte-aligned. Valid memory types are listed later in this chapter.

Producer and Consumer Period Ranges

Producer and consumer periods may be configured to be between 10 milliseconds (2 milliseconds for the Series 90-30 CPU374) and one hour (inclusively). They should be configured in increments of 10 milliseconds (2 milliseconds for the Series 90-30 CPU374). If not, they are rounded to the next highest 10-millisecond increment during operation (2 milliseconds for the Series 90-30 CPU374). For example, a configuration value of 11 (intended to be 11 milliseconds) will be treated the same as 20 ms by the Ethernet Interface (12 milliseconds for the Series 90-30 CPU374).

For the Series 90-30 CPU374, you may also configure the producer period with the value of zero. A zero period is interpreted as producing the data "as fast as possible". For the CPU374, zero period production is scheduled every scan or every 2ms, whichever is slower. In a PLC with rapid scan times, scheduling a produced exchange at zero results in a very high load on the network and on the Ethernet Interface, which may degrade overall Ethernet performance. Scheduling multiple exchanges for a zero period in a PLC with a low scan time can result in the Ethernet Interface being unable to produce all the required data.

Effect of Enabling User Interrupts

If user interrupts are enabled (applies to Series 90-70 only), the data values of a variable greater than 256 bytes are not guaranteed to be “atomic” (or “coherent”, meaning their snapshot is taken at the same instant). If atomic data transfer is desired for a variable within the exchange, disable user interrupts or limit the size of the variable to 256 bytes or less.

Update Timeout Period

Set the update timeout period for the consumer to twice the producer period and at least 20 milliseconds greater than the consumer period. Otherwise, the PLC may occasionally falsely report refreshment faults. Use zero for the update timeout period of a consumed exchange to disable timeout detection.

GFK-1541B Chapter 5 Ethernet Global Data 5-7