4 Transmitter start up
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266 MODBUS® multivariable transmitter User manual 2105216 Rev. AB
Set MODBUS
®
device address
The MODBUS
®
address can be set or modified using any one of the following methods:
The HMI can be used to directly change the MODBUS
®
address to the application requirements. See Appendix A,
HMI menu structure (page 79), for the menu structure.
If a single device is connected to the MODBUS
®
Master, the device address can be modified by writing the correct
MODBUS
®
address to MODBUS
®
register 503. The device supports the global MODBUS
®
write sequence (MODBUS
®
Address 0 writes).
This method allows for plug/play capability into an existing system. A special set of MODBUS
®
operations are used
to change the device address. The MODBUS
®
Master performs the simplest part of these operations, it just has to write to
the Transducer ID / Serial number register using the device address that it wants to assign. However, the value written must
match the device’s Transducer ID for the device to accept the assignment and acknowledge the command. The Master has
three out comes after making the request:
-
The Slave answers correctly. The Master changed the device address successfully.
-
The Slave did not answer and the communication timed-out. The Master did not change the device address,
leaving the device without a Transducer ID or an identification on the bus.
-
The Slave replied with a negative acknowledgement (NACK) and the device address did not change. The Master
could not reassign the device address because the device address already exists, but does not match the
Transducer ID of the device. Reassigning the device address to an existing address would create a duplicate,
therefore no device address change occurred.
To use Method C, the MODBUS
®
Server state processing of request packets needs a couple of adjustments to allow this to
work. The shaded areas are the adjustments.
In Figure 32,
MODBUS® Server state diagram
, the “Request Process” must perform a custom "put" operation. The operation
compares the data buffer received to the target device Transducer ID. If the data matches, the device modifies the MODBUS
®
address register to the device address sent in the request frame. NOTE: The data in the Transducer ID register remains
unchanged.
Entry
[2] Function code
Supported
No
Yes
[3,4] Start Register Address
And
Start + Quantity [5,6]
Valid
Yes
[1] My Address
OR
(Function == 16 AND Start Address ==
Transducer ID)
Yes
Request Process
Yes
MB Server Sends mb_rsp
No
Exception 1
No
Exception 2
No
Exception 4
My Device Address
Yes
MB Server Sends mb_exception_rsp
Exit
No
Figure 32: MODBUS
®
Server state diagram
Summary of Contents for 266 MODBUS
Page 163: ......
