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1771sc-IFE32 32-Channel Analog Input Module Configuration with PLC-5 ControlNet Systems Page 6 of 7

Spectrum Controls, Inc Phone: (425) 746-9481

P.O. Box 5533 Fax: (425) 641-9473

Bellevue, WA 98006 Web Site: www.spectrumcontrols.com

Note that the Requested Packet Interval (RPI) for this module has been set to 80 milliseconds. If the RPI is set

extremely low, the module will not be able to respond to the ControlNet adapter’s data transfer requests and a

connection error will result.

For example, for the default ControlNet system Network Update Time (NUT) and default RPI of 5 milliseconds, the

default analog module RPI is 4x the NUT, 20 ms. This is too fast for the module to handle.

RPI’s must be set as binary multiples (x2, x4, x8, x16, x32, x64, and x128) of the NUT. For a default NUT of 5

milliseconds, the binary multiples are 10, 20, 40, 80, 160, 320, and 640 milliseconds.

Input data update timing.

The 1771sc-IFE32 module will make data available to the 1771-ACNR15 network adapter only after it has scanned

all of it’s input channels. This time can vary depending upon the filtering settings for each channel, and upon the

usage of Real Time Sampling. Consult the User Guide for details about these features.

When using unscheduled messaging, a request for data will be made from the module only when the CIO 1771

Read instruction is executed. If the module does not immediately respond to the request, the adapter will wait until

it does, then return a reply to the CIO 1771 Read.

Although it is impossible to predict how fast a CIO instruction will complete (because it is using the unscheduled

portion of the ControlNet bandwidth), test systems have returned CIO instructions in under 10 milliseconds on

lightly loaded ControlNets.

An efficient way to program CIO instructions is to have the instruction repeat only after the Done bit has indicated

the instruction has received a response. Because CIO instructions have no built-in timeout, error handling and

watchdog logic may be required as well.

The worst-case update time with such a method will be the module update time plus the time required for the CIO

instruction to complete.

When using scheduled messaging, requests for data will be made by the 1771-ACNR15 from the IFE32 module

continuously and that data will be exchanged with the PLC-5C15 controller at the rate set by the RPI in RSNetworx.

If the RPI is set, for example, for 100 milliseconds and the module takes 500 milliseconds to update all of it’s

channels, 5 identical copies of the module data will be transferred to the controller.

With scheduled messaging, the worst case situation is when the RPI falls just before the completion of the module

update. In such a case, the total update time will be the module update time plus the RPI. The most predictable

update times will be achieved using Real Time Sampling and scheduled messaging. A common rule of thumb is to

set the RPI to 3 or 4 times faster than the calculated module update time.

Summary:

The 1771sc-IFE32 may be used in PLC-5 ControlNet systems with only small modifications to the ladder logic,

replacing BTR and BTW block-transfer instructions with CIO 1771-Read and 1771-Write ControlNet I/O Transfer

instructions.

In addition, the module input data from the 1771sc-IFE32 may be part of the scheduled I/O configuration for the

PLC-5 based ControlNet network. In such a system, the module still receives it’s channel configuration data via a

CIO instruction.

Update time for the 1771sc-IFE32 input data is dependent principally on the module’s configuration or RTS

settings, but may also be affected by the data transfer method used.

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Allen-Bradley PLC-5 Manual de usuario Pagina 6

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