Harper Technologies GPS Display - General and Product Information: ------------------------------------
Copyright © 1995-2001 Harper Technologies. All Rights Reserved.  

GPS Display Products - general info


Some time ago I picked up a surplus Trimble GPS receiver on the local ham radio market. The unit was "headless" (without display), which presented me with the choice of lugging around my laptop all the time or building a small unit to display the data. Well, I happened to have a new little microcontroller chip burning a hole in my pocket. Combined with a 2x16 LCD module and a few hundred lines of code to parse the GGA sentence the receiver was sending, I had a compact unit to show me latlong and time. I showed the unit to various ham radio friends who also had one of these Trimbles and they wanted one as well. I suddenly had a small business making kits for people. By popular demand I added code to compute and display the 6-character Maidenhead Grid Square. (Very handy when I was in Baja for a prior summer's 10GHz contest!)

The KD6PBH GPS Display Unit

The KD6PBH GPS display is a compact unit which shows latitude, longitude, UTC "zulu" time, the number of satellites and the computed Maidenhead Grid Square location. As input, the unit takes +9-15V power and a serial connection from an NMEA 2.0 compliant GPS receiver (GGA or RMC sentences required). Output is provided in a set format on an LCD display with 2 lines and 16 characters per line and LED backlight. The 6-character grid square computation is valid for any north latitude and west longitude (proper operation is guaranteed for the USA "the 50 states").

The heart of the KD6PBH GPS display is an MC68HC705J1A microcontroller in a 20 pin DIP package. This receives the serial input, drives the LCD display module, and computes the grid square from the latlong. Other components include the smart LCD module, a serial receiver chip, on-board power regulator and LCD bias voltage generator. The RS-232 receiver chip has been shown to work with commercial GPS receivers (see interfacing). The unit consumes 12-15 mA with backlight off and roughly 50 mA with backlight on. Firmware for the micro is burned into on-chip PROM, and upgrades are accomplished by replacing the microcontroller. 

NMEA Sentences used - GPGGA or GPRMC

The original version of this kit parsed only GPGGA sentences for latlong and time. While this works for many receivers (e.g., Trimble, Garmin 45, Tripmate), some receivers do not provide the GGA sentence. These usually do have the GPRMC sentence, which also contains latlong and time. A variant of the display unit is now available which parses the RMC sentence instead of the GGA. Note that this is an "either-or" choice - the micro is programmed to parse one or the other of these sentences. Please specify which you want when ordering).

Please also see New Product Info for product updates and news. 

Format of the output in the 2x16 display

The current output display format is
Bll.ll hhmmssZ
LLL.LL gridsqQ


  • B is a "baton" character which spins to indicate serial line activity.
  • ll.ll is the latitude
  • LLL.LL is the longitude
  • hhmmssZ is the Zulu (UTC) time received.
  • gridsq is the six character Maidenhead grid square computed.
  • Q is an fix "quality" indicator. (For GGA units this is the number of satellites being received (tops out at 9). For RMC units this comes from the RMC field and is A for good fix and V for no fix.)
  • RMC users: The current shipping version of the RMC program also has course and speed display capability. See RMC course and speed update for more information.

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    New GPS Display Products

    Course and Speed display for RMC units (November 1997)

    For quite a while, various RMC users have felt "the need for speed". We're happy to announce the availability of an update of the RMC program which provides course and speed in the display.

    The new data appears in the bottom right of the display, with three characters for course, three for speed and a blank in between. To make room, the fix qualitty indicator moves to the top right, replacing the static "z" of the time (the zulu time reminder).

    In other words, the course and speed display looks like:
    Bll.ll hhmmssQ
    LLL.LL crs spd

    The three digits of the course and speed are the first three digits that your receiver outputs for those fields. The good news here is that whatever modes you can set in your receiver are presented without change. For example, if you can set your receiver to calculate and present MPH, Knots, or KPH in the RMC output, the display will show that information. Similarly, the bearing can often be represented with or without declination adjustments.

    The key here is that the display is showing the results of calculations in the receiver. However, some receivers may output something a little different than what you really want to see. For example, the speed may always be in knots, or the course may not have leading zeroes to always be three digits. We have tested one receiver which outputs 0.0 for both course and speed when stopped. While this behaviour is understandable, it would be more useful to show the last heading. As we cannot test with all receivers in all modes, you may want to review the output of your receiver to make sure that what you can get is suitable for your application.

    The course and speed mode of the display is selected by a jumper on the board which is sensed each time a new RMC sentence is being read. Those that want to switch "on the fly" can simply bring the jumper pins out to a switch. You can also get creative -- how about adding a transistor to ground the pin and switch to course and speed while the car ignition is on and show grid square when stopped? (That could be handy for mobil contesting.)

    If you already have a display kit, you can upgrade to this RMC course and speed capability by ordering only a new CPU. New RMC kits are shipping with improved CPU already, so when ordering a new kit (or wired and tested unit), just specify the RMC controller as usual.

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    General interface considersations and suggestions

    I have been asked many times if one can make a "Y" cable to make the receiver output feed both a TNC and the display. The answer is a rousing "well, usually". In fact, the ability to drive multiple serial receivers with one serial driver depends on the particular drivers and receivers you are using, cabling details, and noise in the environment. While success at this trick is not guaranteed in all situations and configurations, there is a very good chance that you can make it work in your situation. There are several factors which contribute to "making it work".

    Signal levels

    Receivers output their data under some serial line signalling standard. All the units I've seen use unbalanced signalling (meaning that the data pins refer to a common ground), and voltages of under 12V. For GPS receivers, the common levels seem to be 0-5V (NMEA, TTL levels) or +-12V (RS-232), but I'll bet there are some +-7V (RS-423) units out there too. The display kit uses an RS-232 serial receiver to ensure that RS-232 plus and minus 12V input signals will not overload the microcontroller. Happily, the NMEA 0V and 5V signal levels are within the recognition range of RS-232, so receivers which output according to that standard (e.g., the Garmin 45) also work.

    Signal drive or fanout

    The amount of current source or sink capability is also important. The serial specifications require a driver to provide more than enough drive for one input receiver at some line length and speed. In practice, drivers typcially have a safety margin in their capability to drive the line. This safety margin helps support multiple line receivers. Cabling is a factor here, and the best bet is to keep cables short and take reasonable care in cleanliness. In high-RF environments, shielded cable may be a good idea.

    Some empirical results

    I have bench-tested the Garmin 45, a Trimble "mushroom", and the DeLorme tripmate. In my setup, I ran the receiver into the serial port on my PC, and picked off an extra line for the display kit. This worked every time, and I received no garbled or erroneous data at either the PC or the display. My cables were fairly short (ten feet or so), but as they were on the bench I did not take any care to make them clean. Transmitting on VHF and UHF radios installed on the same bench produced no noticable problems (the antennas are a few yards overhead).

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    Specific interface info for DeLorme Tripmate

    The Tripmate and Street Atlas 4.0 bundle

    Tripmate is a new low cost receiver which has received a great deal of attention in the ham radio community. Tripmate consists of the receiver bundled with DeLorme Street Atlas 4.0 (SA4) software for windows. DeLorme only guarantees the receiver to work with SA4, and SA4 has a short list of other acceptable receivers. Hams willing to void their warranty have tinkered enough to produce ways to hook the Tripmate up to their TNCs for mobile trackers. There are many detailed reports of the considerations on the net and on the TAPR APRS SIG.

    Battery conservation requires DTR

    The Tripmate recevier requires an RS-232 signal called DTR (Data Terminal Ready) to wake up and operate. DTR normally indicates that there is software running on the machine which is using the serial port - presumably SA4. As the Tripmate normally works from internal batteries, this is a great way to save battery power. When connecting the Tripmate to your TNC or another device (say, a display unit), you need to arrange for DTR to be asserted on aserial port pin. This requires a voltage between +4 and +12.

    Startup command

    In addition, the Tripmate will not actually start sending GPS data until it receives a command to do so on the serial input. Various people have worked out what the command string is, and in fact have found that an acceptable command is output by Tripmate as it starts up. Looping back the Tripmate transmit pin to the receive pin does the trick.

    The empirical side

    Back to the work bench. I happen to use a DB-9 connector on my display kit. I modified that for direct connection to the Tripmate (both use nominally standard DB-9 RS-232 pinouts). All I needed to do was loop TxD to RxD (pins 2 and 3) and put +5V on DTR (pin 4). The display kit has on board +5V regulation, which I used as the source of voltage for DTR. I powered up, the "baton" started going around, and in a few minutes I had a fix. (Note that I also had a "Y" cable running to my PC running hyperterminal so I also could watch the output.)

    CAUTION: Do NOT use an unregulated source, such as your car's electrical system, for DTR. Car voltages usually exceed the RS-232 specification of 12V by 20-30% (14-15V).

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    GPS Display Products and Ordering

    Please note: Due to changes in technology, we are out of kits at this time.
    We cannot promise a fixed timeframe for delivery.

    Please email to inquire about availability and/or lead times.


    The kit includes all parts, a PC board, programmed microcontroller, instructions, and the LCD module. Not included in the kit are the external cabinet and connectors for your application. Several folks have used the blue plastic Radio Shack box measuring roughly 1.25 x 2.5 x 4.5 inches, with Molex and/or DB-9 connectors.

    We also offer assembled kits for an additional $25 each. Wired and tested units exclude the external cabinet and connectors.

    We also offer CPU upgrades. This consists of a new microcontroller programmed with the current revision of firmware. (So all you folks who already have a kit can get the latest and greatest features without buying a whole new kit.)

    Shipping and handling charges are required on all orders. Orders will be shipped Priority Mail for the stated rates. Other carriers and services (e.g., overnight and international) will be quoted individually - contact me by email.
    Note: There is one exception to shipping charges. I will not charge shipping/handling on kits delivered to a ham club meeting which I often attend. I frequent Palomar ARC, and SD Microwave Group meetings; if you would like to meet at one of those to take delivery, please contact me in a few days in advance to verify that I will be there and can have a kit ready for you.

    Sales tax is required for California residents, and must be included in your order. Please see below for charges.

    Basic display unit

  • Kit (including one GGA or RMC CPU) - $45
  • Wired and tested kit - $70
  • Note: Specify RMC or GGA when ordering.
  • CPU Upgrade - (feature update for those with kits already)

  • RMC or GGA programmed microcontroller - $10
  • Note: Specify RMC or GGA when ordering.
  • CA residents: Apply tax to all orders.

  • CPU only tax $0.75,
  • Kit tax $3.50,
  • Wired and tested kit tax $5.50.
  • (This is an average of about 7.75%, which enables me to quote a single price across the state.)
  • Shipping and handling:

  • Orders under $25... add $1.
  • Orders $25 to $125... add $5.
  • Orders over $125... add $10.
  • Please note:
    Due to changes in technology, we are out of kits at this time.
    We cannot promise a fixed timeframe for delivery.

    Please email to inquire about availability and/or lead times.

    Please mail orders and payment to:

    Harper Technologies
    P.O. Box 26910
    San Diego, CA

    Delivery normally takes four to six weeks. (While we can often ship from stock, this is not always possible. If you need an order in a specific timeframe, please email. a request with your needs so that we can work something out.)

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    Links to Related Information

    Peter Bennett's GPS info page A wealth of GPS info, programs, etc.!
    Tucson Amateur Packet Radio TAPR is on the forefront of Ham Radio digital work
    DeLorme Mapping Home page about DeLorme products and Services, including Tripmate
    GPSy  Home page for cool Mac GPS software and custom data/power cables

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    Copyright © 1995-2001 Harper Technologies. All Rights Reserved.