GE GFK-2193A user manual Operation of the Communications Request, Plc

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FT Output: The function’s FT (fault) output can provide an output to optional logic that can verify successful completion of the Communications Request. The FT output can have these states:

FT Output Truth Table

Enable Input Status	Does an Error Exist?	FT output
Active	No	Low
Active	Yes	High
Not active	No execution	Low

The FT output is set High if:

￿The specified target address is not present (for example, specifying Rack 1 when the system only uses Rack 0).

￿The specified task number is not valid for the device.

￿Data length is set to 0.

The FT output can either be connected to another device, such as a set coil, or can be left open.

Operation of the Communications Request

The figure below illustrates the flow of information between the PLC CPU and the

PROFIBUS Slave module:

PLC CPU

Ladder

program

•COMMREQ

CPU

memory

•Data

•Status word

PLC

Backplane

Command

Requested data

Status bits

PROFIBUS Slave

Module

Firmware

instructions

On-board memory

PROFIBUS

network

to Master

Figure 4-2. Operation of the PROFIBUS Communications Request

A Communications Request is initiated when a COMMREQ ladder instruction is activated during the PLC scan. At this time, a command from the PLC via the Communications Request is sent to the PROFIBUS Master module (PBM).

At the conclusion of every request, the PLC CPU reports the status of the request to the Status Word, which is a location in PLC memory that is designated by the Status Word Pointer in the Command Block.

In Figure 4-2, the PBM is shown in the CPU rack and communications occur over the PLC backplane. If the PBM is located in an expansion or remote rack, the commands and data are sent over the CPU rack’s backplane, through the expansion or remote cable to the rack containing the PBM, and across that rack’s backplane to the PBM.

4-4 Series 90™-30 PROFIBUS Slave Module User's Manual – August 2004 GFK-2193A