Owner's manual

8
2.3 - Modbus Slave Operation
When confi gured as slave, the 7231 HT responds to
requests for data from a Modbus master. The data
available is the product, interface, total covered tem-
perature, product temperature, interface temperature,
span (probe length), max temperature, and the individ-
ual values of the 5 temperature sensors from the probe
as well as the status of the probe.
The 7231 HT is capable of communicating using the
Modbus RTU or ASCII format. The format is selected
by the on board jumper JP2. The serial communication
parameters are selected by the on board jumper JP3.
The baud rate and Modbus device number are selected
by on board switches. Refer to the table and fi g. 2.1 on
pg. 9 for the jumper settings.
Individual data items (product, average temperature,
etc.) are obtained by issuing a Modbus request with a
specifi c register number, as per the Modbus protocol
specifi cation. Multiple values can be obtained by speci-
fying a starting register and the number of registers
desired.
The data returned from a Modbus request can be in
many formats: 16 bit integer, 32 bit integer (long), or
fl oating point. The 7231 HT has a unique feature which
allows the user to specify the data format desired by
the selecting the correct register. For example, the
value for the product reading can be obtained as an in-
teger (16 bits), a long word (32 bits), or a fl oating point
value (32 bits) by simply choosing different registers.
In addition to the length of the data, the order of the
bytes in the returned data must be known by the
Modbus master in order to assemble the data
correctly.
The 7231 HT allows for all byte ordering combinations
by specifi cation of the correct register number.
In traditional Modbus, 1 register represented 16 bits of
data. To obtain a 32 bit value, the master requested 2
registers to get the 32 bits. A variant of Modbus called
Enron Modbus allows for 32 bits of data to be returned
with 1 register request. The 7231 HT will work with
either type of Modbus master. Again, the format type is
controlled by the register number requested.
Switch
position
Baud (in bps)
0 Sets 7231 HT as a Modbus master
1 600
2 1200
3 2400
4 4800
5 9600
6 14400
*7 19200
8 38400
9 57600
2.4 - Baud Rate
The baud rate for a Modbus slave confi guration is
determined by the Baud rate switch, summarized in the
following table:
Basic
Addressing Confi gurable address from 1 to 247
Broadcast Yes
Baud Rate 600 to 57,600 bps
Mode RTU/ASC II
Parity Even, None
Stop Bits 1 if Even Parity, 2 if No Parity,
Electrical Interface RS-485 2W-Cabling
Connector Type 3 Wire Terminal
Implementation Class
The 7231 uses the Basic Implementation Class of the
Modbus protocol. The table below shows confi gurations
capabilities of the basic implementation class.
* Default Setting