Modbus - RTU Setup
Modbus-RTU Se ngs 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 RS-485 MicroLink-HM. The Modbus address
can be set to any value from 1 to 247. The RS-485 MicroLink-HM
con
fi
guration software will discover this address as part of the
connection process.
Baud Rate & Parity
The RS-485 MicroLink-HM baud rate and parity must be set to the
same settings as your Modbus master. If you are using the RS-485
MicroLink-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 applica on. Register maps can be viewed
using the RS-485 MicroLink-HM con
fi
gura on so ware. 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 frac onal informa on, 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 e
ff
ect of the decimal place se ng. Decimal place
se ngs are not used for the Modus
fl
oa ng point register values.
Figure 6. Modbus se ngs dialog.
Figure 7. Menu for viewing modbus register maps.
Floating Point Byte Order
Modbus 32 bit
fl
oa ng point numbers are stored in two
consecu ve 16 bit registers using the IEEE-754 standard
big endian byte order (AB-CD). The most signi
fi
cant byte
(A) is sent
fi
rst. For compa bility 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
communica ons errors, it will retry for the number of
mes set in the HART Protocol Se ngs dialog. If a er
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
oa ng point value. This can help the
modbus master determine when a HART device is no
longer responding.
