Modbus - RTU Setup
Modbus-RTU Settings can be accessed and changed by clicking
Setup>Modbus
from the top drop-down menu.
Modbus Address
Each device on a Modbus network must have a unique address. Set
this to match the address that your Modbus master will use to com-
municate with the MicroLink-HM. The Modbus address can be set
to any value from 1 to 247. The MicroLink-HM confi guration soft-
ware will discover this address as part of the connection process.
Baud Rate & Parity
The MicroLink-HM baud rate and parity must be set to the
same settings as your Modbus master. If you are using the Mi-
croLink-HM as a HART protocol modem these should be set to
1200 baud, odd parity. Click
Use HART Protocol Settings
to set the
correct values for a HART modem.
Holding Registers Map
Accumulated data is stored in Modbus registers that are grouped or
mapped by HART device or by variable type. Choose the register
map that best fi ts your application. Register maps can be viewed
using the MicroLink-HM confi guration software. From the top
drop-down menu select
View -> Modbus Registers Maps
(Figure 7.)
Decimal Places
When device data is stored into a 16 bit signed Modbus register the value can range from -32768 to 32767. To allow the
16 bit integer to represent a smaller number and include fractional information, the number of decimal places is stored in
a separate Modbus register and applied later. The number of decimal places can be set from 0 to 5. Decimal places can
be set for each variable. The
Device Variables
box shows the eff ect of the decimal place setting. Decimal place settings are
not used for the Modus fl oating point register values.
Figure 6. Modbus settings dialog.
Figure 7. Menu for viewing modbus register maps.
Floating Point Byte Order
Modbus 32 bit fl oating point numbers are stored in two
consecutive 16 bit registers using the IEEE-754 standard
big endian byte order (AB-CD). The most signifi cant byte
(A) is sent fi rst. For compatibility with some modbus
systems you may need to use the word swapped format
(CD-AB).
HART Device Failed Register Value
If a polled HART device fails to respond, or has commu-
nications errors it will retry for the number of times set
in the HART Protocol Settings dialog. If after retries the
device does not respond you can choose to hold the last
value, set to a preset number, or set to the IEEE-754 NaN
fl oating point value. This can help the modbus master
determine when a HART device is no longer responding.
