A MultiDisplay Board for Doppler DFs



This section of the website described a sophisticated Doppler display board that displays DF information in both numeric and pelorus formats. I call it the MultiDisplay board, and DF data is applied to the board through a serial RS232 port, using "Agrello DF" messages. It can be used with my own DF, ( if the RS232 board is installed ) or with other Doppler DFs that generate Agrello messages. Straps on the board allow selection of four baud rates. ( 1200 to 9600 )

For the hardcore technoids, I should point out that this board also has prospects as a general-purpose azimuth display board, for other ( non - DF ) applications, such as a compass display or a GPS display. The board employs two PIC microcomputers ( 16C773 and 16F84 ) to achieve almost all the internal functions, so the basic design is very agile, and can be adapted ( with suitable software changes ) to a vareity of other tasks.

This is a nice display board... it has several features and enhancements that make it superior to the original DISPLAY and NUMERIC boards in my DF design, and it can replace both of those boards, if RS232 data is available to drive it.

The mounting holes are designed to fit a SerPac A-27 enclosure, ( available from DigiKey ) which is stylish, inexpensive, and available in black, grey or almond colors. It is also made of soft ABS plastic, so the ( various ) holes for the display and connectors can be "drilled" with a soldering iron and "cleaned up" with a hobby knife. ( crude, but not everyone has access to a drill press or a milling machine. )

If you want to get an idea about the size of this board, ( and the display ) you can download the silkscreen file ( next web page ) and print it out. Then use a photocopier ( with a zoom feature ) to enlarge or reduce it until the board size equals 2.5 x 3.8 inches... The silkscreen file also has the footprint for the mounting holes, so it can also be used as a template.



This board is available as a "bare board" for US$ 50, as a "kit" for US$ 100, or assembled and tested for US$ 150. Cost of new parts to build a single board is about US$ 50. Contact me by e-mail for inquiries, or to purchase: pelican2@silcom.com Discounts for multiple board purchases can be negotiated... contact me.

All the technical information required to build a bare board is provided on the following web page. The ExpressPCB CAD file for the board is NOT available. Source code for the two PIC chips ( written in PIC assembly language ) is also provided on the following web page. If desired, I will provide the ( coded ) 16C773 chip for US$ 20, and/or the 16F84 for US$ 15.



General Profile

The MultiDisplay board employs 36 T-1 LEDs in the pelorus display, ( ten degree bearing resolution ) with yellow LEDs at the cardinal points ( north, east, south and west ) and red LEDs at all other points. The 3-digit numeric display employs red 7-segment readouts with a character height of 0.36 inches.

When power is applied, the display goes through a "self-test" routine which "orbits" the pelorus LEDs, ( clockwise ) and tests all the numeric readout segments, for a few seconds. After that, ( if no data is arriving ) the Pelorus display will continue to "orbit", ( about twice each second ) to indicate that no data is available, and the numeric readout will display horizontal dashes.

When data arrives through the RS232 port, it is tested, decoded and displayed on both the pelorus and numeric readouts. Each pelorus LED has a 10-degree "window" and illuminates for bearings ranging from -5 to +4 degrees. For example, a bearing of 355 to 004 degrees will illuminate the LED at the 12 o’clock position of the display. ( = 000 degrees )

For the benefit of nighttime viewing, the entire pelorus is "flashed" for a few milliseconds, ( about once per second ) to provide an azimuth "context" for the ( single ) LED that is currently indicating the signal direction.

If data is interrupted, ( loss of signal, garbled data, etc. ) the display will hold ( and display ) the last valid data for about three seconds, then resume "orbiting". A strap allows the option of "freezing" the last valid data indefinitely, in which case the display will "blink" about twice per second to indicate the display is not updating.

The MultiDisplay board can be used with "standard" Agrello messages, and with the "extended" Agrello messages which are only generated by my own DF design. If the extended Agrello message is employed, a switch input allows the display to indicated bearings either in "relative" degrees, ( 12 o'clock = vehicle heading ) or in north-stabilized "magnetic" degrees. ( 12 o'clock = magnetic north ) The "North stabilized" display is somewhat better for all but the terminal phase of the hunt, ( easier to interpet ) and also facilitates plotting a bearing line on a map.

The RS232 input has a fairly high input impedance ( 10K ohms ) so it can be attached "in parallel" with any pre-existing RS232 line, without disturbing the original data link. Wire pads are provided for the RS232 connector, which is external to the board itself... there just wasn’t room on the board for it.



Technical Profile

The board meaures 2.5 x 3.8 inches and was designed using the ExpressPCB "mini board" web service. It is a high quality, commercially manufactured board, with through-plated holes and a tin-lead reflow finish. Very nice...

To conserve board space, two microcomputers are employed... one is a PIC 16C773, which contains the RS232 UART. It also drives the 36 LED pelorus display, and generates a 2-wire serial output to drive the second PIC micro, which is a 16F84. The second PIC micro decodes the data received over the 2-wire interface and drives the 3 numeric readouts.

The 36 pelorus LEDs are arranged in a 6 x 6 matrix, and the 3 ( common anode ) numeric readouts are time-multiplexed in a 3 x 7 matrix. The pelorus LEDs are directly driven by outputs from the 16C773 PIC chip, but the numeric LEDs require too much current for "direct drive" from the 16F84 chip, so buffer transistors are employed.

An on-board 5 volt regulator ( 7805 ) provides power for all circuits on the board. To maintain a low profile which allows "panel mounting" of the board, the voltage regulator is mounted on the backside of the board. This will allow the installation of a heat sink, should it be required. ( it runs fine at room temperature without it, from a 12 volt source )

Sockets are provided for both PIC chips, so they can ( if desired ) be removed and re-coded by the end-user, to achieve some ( new ) feature or task. The 16C773 is a windowed EPROM device, ( UV eraser required ) but the 16F84 micro employs FLASH memory, and does not require an eraser.