Hmr3100 – Honeywell HMR3100 User Manual

Page 6

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Solid State Electronics Center (800) 323-8295 Page 6

Data Description

The HMR3100’s onboard microcontroller sends a three byte status/heading data packet reply as the RTS line is
brought low. The data is normally formatted in binary with the first byte being either 80(hex) or 81(hex).

If that first byte LSbit is flagged high (81 hex), it means magnetic distortion maybe present and a hard-iron calibration
should be performed. Many end users may choose to ignore this indication in portable applications.

The remaining two bytes are the heading (in degrees) in MSB to LSB format. There is some data interpretation
needed to derive the heading. For example, the 80 02 85 (hex) Byte pattern correlates to 322.5 degrees.

This is done by taking the MSB hex value, converting it to decimal (base ten) representation (e.g. 02 decimal) and
multiplying it by 256. Then the LSB is decimalized (e.g. 85(hex) to 133(decimal)) and added to the 512(decimal) MSB.
The total (512+133=645) is then divided by two to arrive at a 322.5 degree heading. This data format permits the 0.5°
resolution in two bytes by doing the binary to decimal conversion and division by two.

Development Kit

The HMR3100 Development Kit includes additional hardware and Windows demo program software to form a
development kit for electronic compassing. This kit includes the appropriate HMR3100 Printed Circuit Board (PCB)
module soldered to an intermediate circuit board using a 0.8” spacing pin arrangement. The intermediate board
assembly plugs into an RS-232 motherboard with a serial port connector. In addition, a four-foot serial port cable (RJ-
11 to D-9F), nine-volt battery clip, demo program software, and user’s guide is included. The RS-232 motherboard
incorporates a 5-volt regulator integrated circuit to provide the necessary voltages to the onboard RS-232 converter
integrated circuit and the HMR3100 daughter-board. A nine-volt battery clip is included, but other DC input voltages
between 7 and 15 volts may be used. Supply currents are nominally around 8mA plus the HMR3100 current draw.

The RS-232 motherboard also contains a six-contact modular jack (RJ-11) for a compact RS-232 interface to a
personal computer serial port. Ground, RTS, RXD, and TXD data lines are brought out to the jack with two contacts
left open. The demo software stimulates the RTS and RXD lines and reads the data from the TXD line for graphical
display on the host computer. No other support software is available. Figure 5 shows the kit board assemblies.

Figure 5

HMR3100 Kit Hardware