Finding the COM Port Number
You will need to know the COM port number the RS-485 MicroLink-HM modem is connected to. To see a list of COM
ports installed on your PC, open the Device Manager and select View > Devices by Type. Expand Ports (COM & LPT) to
see the list of available COM ports. RS-485 MicroLink-HM is compable with RS-485 to USB or RS-232 serial converters.
HART Protocol Modem
The RS-485 MicroLink-HM ships configured to operate as a HART protocol modem. Connect the A and B terminals to
an RS-485 port and provide a power supply. Power must be supplied by an external 5 to 30 volts DC power supply.
Refer to the Power Supply secon in this manual for details. If your PC does not have an RS-485 port you will need to
use an RS-485 converter. Microflex offers the 101-0020 USB to RS-485 converter and the 101-0009 RS-232 to RS-485
converter.
RS-485 MicroLink-HM is compable with most HART configuraon and test soware. Included on the CD is the
PACTware FDT frame, HART Protocol DTM, and Microflex Generic DTM soware. For informaon on installing PACTware
and the DTM, please reference the PACTware Quick Start Guide.
To funcon as a HART protocol modem the serial port is typically set to 1200 baud, odd parity. Most HART soware
will require this seng. It is possible to set the serial port to other baud rates and parity sengs. This is helpful when
connecng the RS-485 MicroLink-HM to a device that does not offer 1200 baud. In this case RS-485 MicroLink-HM will
receive the HART command at the higher baud rate then transmit the HART data at 1200 baud, odd parity onto the
HART loop. The 1200 baud, odd parity HART reply packet is received by the RS-485 MicroLink-HM modem and then
transmied back at the higher baud rate.
If the RS-485 MicroLink-HM is configured to Fill Modbus Registers and it detects a HART packet on the network, RS-485
MicroLink-HM will temporarily switch to HART mode and stop polling devices. HART protocol packets always begin
with the hex FF preamble character. Modbus packets begin with the slave address. If no HART commands are received
over the serial port, RS-485 MicroLink-HM will return to polling and filling modbus registers aer 30 seconds. Also, if a
modbus command is received MicroLink will exit HART mode and return to polling and filling modbus registers.
Connecting to the HART Loop
Connect the two screw terminals to the HART device
or HART loop. Microlink provides electrical isolaon
between the HART loop and the serial port so it is safe
to ignore grounding and polarity issues when making
the HART connecons. The HART protocol requires a
loop resistance of 230 to 600 ohms, typically 250 ohms.
Refer to your equipment installaon instrucons for
details on connecng a HART master or configuraon
device to the loop.
HART Status LED Operation
The LED on the front panel of the RS-485 MicroLink-HM
is used to monitor HART communicaons acvity. The
LED will be green while transming or sending a packet
to your HART protocol devices, and will be red when
HART protocol replies are received.
LOOP
POWER
SUPPLY HART
DEVICE
250Ω
LOOP
POWER
SUPPLY HART
DEVICE
250Ω
Figure 1. Connecng HART modem to device loop.
Connecting across the loop resistor.
Connecting across the HART device.
