Status Instruments DM4000U, Manual

Page: 84 / 94

The other problem obvious from the above schematic is that even though the
MASTER transmits to all of the SLAVES simultaneously, only one may respond,
otherwise signals will clash together. This is arranged by allocating each SLAVE unit a
unique address. This address is called the device number and is programmed into the
unit before it is connected to the network by programming the CO NET parameters
menu within the SYS menu. See the programming section for details.
The comms messages issued by the host can be one of two types.
Data Request - MASTER requests data from a SLAVE.
Data Imposition - MASTER writes data to a SLAVE.
The most convenient way to describe these message types is to show an example for
each message type. It is not important to understand the full detail of the message at
this stage as this will be covered later, however the following control code definitions
will probably be useful.
<SOH> Start Of Header
<STX> Start Of Text
<ETX> End Of Text
<ACK> Acknowledge
<BCC> Block Check Character
For the purposes of this example assume that the MASTER is connected to three
SLAVE units as shown in the above schematic. The Device numbers for the units are
1,2 and 3 and device number 2 has an Thermocouple input measuring 79.8 degrees C
and an Alarm Output.
APPENDICES
APPENDIX E
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