Adding RS232 to the Wide Band FMD

Further simple modifications to the modified Fuel Mixture Display kit (the WB FMD) allow it to generate an RS232 data stream that can be displayed and logged on a laptop, or other recording device. All modified KC-5300 kits shipped since early December 2001 have version 1.1 code that includes the RS232 code that is enabled when the following hardware changes are made (note: the version number is displayed when the WB FMD first starts up).

The current software produces a single byte of data at 19,200 baud, at a repeat rate of about 220 milliSeconds (that is, each time the bar graph is updated). The byte sent out is the current AFR times 10, so that an AFR of 14.7 will produce a byte value of 147 or $93 hexadecimal.

Hardware Changes The circuit is quite simple and uses an NPN transistor as both an AND gate and an RS232 driver to produce an output between 0 and +12 Volts. Although not true RS232, this is adequate to drive the vast majority of RS232 receiver chips. Normally RA0 and RA1 are never driven low together as they control which 7 segment display is active in the multiplexed display scheme used. The NPN transistor will normally be conducting due to the base current through one or both of the 22 k resistors. Its collector will normally be at a low voltage. If the software turns all segments OFF (by driving RB1 through RB6 high), then if RA0 and RA1 are driven low, the collector of the transistor will be driven high (collector current off). In this way the software can toggle the voltage levels at the collector of the transistor and produce an RS232 signal. The high baud and low repetition rate of the signal ensure the displays are never off for more than a few percent of their maximum duty cycle. Note that a BC337 is included in the KC-5300 kit and is otherwise unused when the WB modifications are made. A 2N3904 may be substituted if you cannot find the original BC337, but note that the 2N3904 transistor has reversed oriented compared to the BC337. Almost any general purpose NPN transistor could be used in this application, as long as it is wired correctly.

The modifications are all made on the processor PCB. You should drill a small hole to anchor the collector (C) of the transistor and the left hand side of the 4.7 k resistor. The hole must be between two existing traces on the solder side of the PCB. The other end of the 4.7 k resistor goes to the existing hole for the 12 Volt connection. The base (B) wire from the transistor is bent up and soldered to the right hand end of the two 22 k resistors. The other ends of the 22 k resistors are soldered to the two existing 680 ohm resistors. The emitter (E) of the transistor goes to the existing hole for the chassis ground connection. You should use a female DB9 connector (DB9S, S=socket) so that the WB FMD can connect directly to a laptop, or could be connected via a standard serial cable (not a null modem cable). Just two wires are needed to the DB9 - pin 2 for the data (goes to the laptop's Rx input), and pin 5 for GND.

PC Based Logging Software

I have written a preliminary version of FMDLOG, a DOS/PC based program that displays the value received from the WB FMD, and also formats it into a text string that is written to a disk file that can later be analysed using eXcel or another display program. You will be able to download this software soon.

A more feature packed Windows version is planned and will be made availible as time permits.

Obtaining the PIC RS232 FMD Firmware

The modified PIC based RS232 firmware (version 1.1 or later) will only be available:

• as a preprogrammed PIC chip when a KC-5300 is ordered through Tech Edge Pty. Ltd.
  
  
• as a HEX file that can be used to program a blank 16F84A PIC. Requests for the hex file should be directed to peter@techedge.com.au with a note that you will not use the software for commercial gain.
  
  
• I will program your own PIC for free if a brand new fully functioning 16F84A is sent to me along with a stamped self addressed return envelope (in Australia), or with the equivalent of US$3.00 to cover postage and packing if from outside Australia. Ensure your package will not be damaged in transit and you must supply a mailing label with your address.

This software is copyright © 2001 Tech Edge Pty. Ltd. and I reserve the right to refuse any individual or organisation the supply of this software for whatever reason. In general the software will be made freely available if you want to use the FMD with the DIY-WB kit and you are doing this for your own personal non profit use. If you are a commercial organisation, then it may be possible for you to licence the software from Tech Edge Pty. Ltd for your own purposes. We can modify the software or the hardware design, and can do so at out normal consultancy rates.