Finding the COM Port Number
You will need to know the COM port number the 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. MicroLink-HM is compatible with USB to serial converters as well as RS-232 serial ports.
HART Protocol Modem
MicroLink-HM ships confi gured to operate as a HART protocol modem. No additional drivers or external power sourc-
es are needed. Simply connect the 9-pin D-sub connector to a standard RS-232 port. Power can be supplied by the DTR
handshake line or by an external supply. Refer to the
Power Supply
section in this manual for details.
MicroLink-HM is compatible with most HART confi guration and test software. Included on the CD is the PACTware
FDT frame, HART Protocol DTM, and Microfl ex Generic DTM software. For information on installing PACTware and
the DTM, please reference the PACTware Quick Start Guide.
To function as a HART protocol modem the serial port is typically set to 1200 baud, odd parity. Most HART software
will require this setting. It is possible to set the serial port to other baud rates and parity settings. This is helpful when
connecting the MicroLink-HM to a device that does not off er 1200 baud. In this case 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 MicroLink-HM modem and then transmitted back at the
higher baud rate.
If MicroLink-HM is confi gured to Fill Modbus Registers and the serial port detects a HART packet on the RS-232 port,
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, MicroLink-HM will return to polling and fi lling modbus registers after 30 seconds. Also, if a mod-
bus command is received MicroLink will exit HART mode and return to polling and fi lling modbus registers.
Connecting to the HART Loop
Connect the two screw terminals to the HART device
or HART loop. Microlink provides electrical isolation
between the HART loop and the serial port so it is safe
to ignore grounding and polarity issues when making
the HART connections.
The HART protocol requires
a loop resistance of 230 to 600 ohms, typically 250
ohms.
Refer to your equipment installation instructions
for details on connecting a HART master or confi gura-
tion device to the loop.
HART Status LED Operation
The LED on the front panel of the MicroLink-HM is
used to monitor HART communications activity. The
LED will be green while transmitting or sending a pack-
et 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. Connecting HART modem to device loop.
Connecting across the loop resistor.
Connecting across the HART device.
