GE GFK-0804B manual Message Types, Transmission Sequence, Query, Broadcast

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Message Types

There are four message types: query, normal response, error response, and broadcast.

Query

The master sends a message addressed to a single slave, such as a Micro PLC.

Normal Response

After the slave performs the function requested by the query, it sends back a normal re- sponse for that function. This indicates that the request was successful.

Error Response

The slave receives the query, but for some reason it cannot perform the requested func- tion. The slave sends back an error response which indicates the reason the request could not be processed. (No error message will be sent for certain types of errors. For more information see Communication Errors, page C-18).

Broadcast

The master sends a message addressed to all of the slaves by using address 0. All slaves that receive the broadcast message perform the requested function. This transaction is ended by a time±out within the master.

Transmission Sequence

The master begins a transmission by sending a query or broadcast request message. If the master sent a query, the slave sends a normal or error response.

If master sends a broadcast request, the slaves do not send responses.

Query Transaction

Master

Slave

Broadcast Transaction

Master

Slave

Slave Turn±around Time

aGGGGGGGG'

Query Message

Response

Broadcast Message

(No Response)

The time between the end of a query and the beginning of the response is called the slave turn±around time (see above). This varies, depending on the query and the activ- ity of the Micro PLC application program. 500mS is a reasonable worst±case estimate.

C-2 Micro PLC Programmer's Guide ± April 1994 GFK-0804B