HART Protocol Settings
With
Fill Modbus Registers
unchecked, RS-485 MicroLink-HM will func on as a standard HART protocol modem and
not poll HART devices to
fi
ll Modbus registers. In this mode HART packets are received and then retransmi ed at the
HART 1200 baud, odd parity. Received reply packets are retransmi ed at the selected baud rate. RTS ming and carrier
detect are handled internally by the RS-485 MicroLink-HM allowing
RS-485 communica ons at higher than 1200 baud. HART loop data
is handled at the standard HART protocol 1200 baud, odd parity
but because RS-485 MicroLink-HM bu
ff
ers the data, it can be set for
higher rates as well as odd, even, or no parity. Port se ngs can be
set in the Modbus Se ngs sec on.
Since RS-485 MicroLink-HM manages network ming it needs to
be con
fi
gured for either a primary or secondary master. Portable
con
fi
gura on modems are typically set to be secondary masters.
Polling HART Devices - Fill Modbus Registers
To enable Modbus register accumula on, check the
Fill Modbus Registers
op on. The Hart Protocol Se ngs window will
expand to show the Polled Devices se ngs.
Retries
If a HART devices is polled but does not respond, or errors are detected, RS-485 MicroLink-HM can retry up to 3 mes
before indica ng a bad device. If a device is not responding the Modbus values are
fi
lled with Hart Device Failed
register values - setup is done in the Modbus con
fi
gura on sec on.
Polled Devices
In the example screen shot (Figure 5), RS-485 MicroLink-HM is set to poll one HART device using polling address 0 and
will retry 1 me before repor ng a communica ons error. The
Device Variables (Modbus Register Values)
box on the right
displays the register values for the ac ve polled device. Click
Add Device
to add a polled HART device. Be sure to set
the
Polling Address
for each device.
Each device must
have a unique HART
polling address.
Polling addresses
must be setup for
each HART device
before it can be
connected to a
mul -drop HART
loop and can be set
from 1 to 63.
Address 0 can only
be used when one
device is in the HART
loop.
Polling addresses
16-63 should only be
used with HART re-
visions 6 and higher.
For HART 3-5 use
addresses 1-15 for
multi-drop systems.
Figure 5. HART protocol se ngs dialog when
fi
lling modbus registers is enabled.
