Cropico DO6, Handbook

Page: 51 / 107

DO6 Handbook
Jan 2006 51 of 107 pages
Or
2. Fast selection
In this case, addressing is combined with the command. This saves a communications
step if you want to exchange data with several devices (via RS485) (see communications
example in section 8.16).
When establishing a connection, the control station can either
•
Specify a slave station
In order to set up a connection i.e. send a command to the addressed slave
Or
•
Poll
In order to relinquish its master status to a secondary station i.e. query for a response to
a previously sent command and hence assign the transmit right to the slave.
7.2.5 Selection with response
The control station sends a “selection supervisory sequence”. The selection supervisory
sequence is used to initialize the DO6 as slave so that it is then possible to send it
commands. The prefix calls up a single secondary station.
<ENQ> defines the end of the
selection supervisory sequence.
The selection supervisory sequence of the DO6 has the following format:
<group_address><user_address>sr<ENQ>
•
<group_address> Group address (decimal, 0 to 99)
•
<user_address> User address (decimal, 0 to 99)
•
sr ASCII characters s and r
•
<ENQ> ASCII character ENQ
A secondary station that recognises its selection supervisory sequence assumes slave status
and sends one of two responses.
•
If the station is ready to receive data it sends back <ACK> . The master station starts
the data transfer on receiving this response.
•
If the station is not ready to receive data it sends back <NAK> . With this response
the master station tries to call up the same station again.
If the master station receives an invalid response or none at all, it can try to address the same
station again or end the transmission.