C
C-2 Micro PLC Programmer’s Guide – April 1994 GFK-0804B
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.
Slave
Query MessageMaster
Broadcast Message
Response
(No Response)
Slave Turn–around Time
Query Transaction
Broadcast Transaction
Slave
Master
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.