HMR3000
HMR3000 supports both standard NMEA 0183 and proprietary messages. Unsolicited NMEA messages are sent by the
HMR3000 in Continuous Mode at the rates programmed in the EEPROM. HMR3000 also responds to all input messages
from the host. An HMR3000 response to a command input may be delayed due to transmission of an unsolicited output.
The host computer must wait for HMR3000 to respond to the last command input before sending another command
message. All communication from and to HMR3000 contain a two-character Checksum Field at the end of the data fields,
and are denoted in the sentences by ‘hh’. The checksum assures the accuracy of the message transmitted. This
checksum is also calculated per NMEA 0183 Standard.
The RS-232 signals are single-ended undirectional levels that are sent received simultaneously (full duplex). One signal is
from the host personal computer (PC) transmit (TD) to the HMR3000 receive (RD) data line, and the other is from the
HMR3000 TD to the PC RD data line. When a logic one is sent, either the TD or RD line will drive to about +6 Volts
referenced to ground. For a logic zero, the TD or RD line will drive to about –6 Volts below ground. Since the signals are
transmitted and dependent on an absolute voltage level, this limits the distance of transmission due to line noise and
signal to about 60 feet.
When using RS-485, the signals are balanced differential transmissions sharing the same lines (half-duplex). This means
that logic one the transmitting end will drive the B line at least 1.5 Volts higher than the A line. For a logic zero, the
transmitting end will drive the B line at least 1.5 Volts lower than the A line. Since the signals are transmitted as difference
voltage level, these signals can withstand high noise environments or over very long distances where line loss may be a
problem; up to 4000 feet. Note that long RS-485 lines should be terminated at both ends with 120-ohm resistors.
Specific measurement descriptions and interface commands are not included in this datasheet but are included in the
companion HMR3000 User’s Guide document.
CIRCUIT DESCRIPTION
The HMR3000 Digital Compass Module contains all the basic sensors and electronics to provide digital indication of
heading and tilt. The HMR3000 has all three axis of magnetic sensors on the far end of the printed circuit board, away
from the connector interface. The HMR3000 uses the circuit board mounting holes or the enclosure surfaces as the
reference mechanical directions. The complete HMR3000 PCB assembly consists of a mother board and the 9-pin D-
connector.
The HMR3000 circuit starts with the Honeywell HMC1001 1-Axis Magnetic Sensor and the HMC1002 2-Axis Magnetic
Sensor elements to provide the X, Y, and Z axis magnetic sensing of the earth’s field. These sensor output voltages are
then amplfied and converted to a digital representation. A microcontroller integrated circuit receives the digitized magnetic
field values (readings) by periodically querying the Analog to Digital Converter (ADC) and performs the necessary offset
value corrections provided by the EEPROM via the calibration routine. This microcontroller also performs the external
serial data interface and other housekeeping functions. The onboard EEPROM integrated circuit also is employed to
retain necessary setup variables for best performance.
A liquid filled two-axis (pitch, roll) tilt sensor is also used to create tilt compensated heading data. This tilt sensor performs
an electronic gimballing function and is normally mounted flat (PCB horizontal) for maximum tilt range.
APPLICATIONS PRECAUTIONS
Several precautions should be observed when using magnetic compasses in general:
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The presence of ferrous materials, such as nickel, iron, steel, and cobalt near the magnetometer will create
disturbances in the earth’s magnetic field that will distort the X, Y, and Z field measurements.
Perming effects on the HMR3000 circuit board need to be taken into account. If the HMR3000 is exposed to
fields greater than 10 gauss, then it is recommended that the enclosure/circuit boards be degaussed for highest
sensitivity and resolution. A possible result of perming is a high zero-field output indication that exceeds
specification limits. Degaussing wands are readily available from local electronics tool suppliers and are
inexpensive. Severe field offset values could result if not degaussed.
NON-FERROUS MATERIALS
Materials that do not affect surrounding magnetic fields are: copper, brass, gold, aluminum, some stainless steels, silver,
tin, silicon, and most non-metals.
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