November 2005, Issue 184

Large-Scale Electronic Display

SOFTWARE

The BASIC Stamp 2 microcontroller’s control program is small. A 3-byte data transfer protocol is used to send data to a digit. The protocol consists of a start byte, an address byte, and a data byte.

The BASIC Stamp 2 microcontroller receives commands from the laptop control program using the SERIN command. The start byte used is a colon, or ASCII 3Ah. To accommodate 25 possible addresses using a single byte, ASCII characters are assigned to digit positions. Digit 0 is character @ (40h). Digit 1 is character A (41h). Digit 2 is character B (42h). After reception, 40h is subtracted from each address byte to recover a digit’s true numerical address value. You may download the code from the Circuit Cellar FTP site.

Similarly, 2Fh is subtracted from each data byte to obtain a numerical data value that will be sent to a digit. This is stored in the DTA variable. The bit pattern sent to a digit is obtained in the manner shown in Figure 4. It uses the LOOKUP command. The DTA variable acts as the pointer in the table. The CHAR variable receives the result. To reset a digit, the / character (2Fh) is used. The t, r, i, and a characters are assigned to the functions shown in Figure 4. For normal operation, :B6 displays 6 on digit 2.

Any terminal program like HyperTerminal will suffice for testing. You can then develop an application program on a PC to control the system via a high-level language.

A student of mine at the University of the West Indies in Barbados wrote the application program in Microsoft Access. The program, which displays a crude replica of the scoreboard, enables you to pull down the appropriate table box and click on a value, which is then sent to the scoreboard (see Photo 2).

My scoreboard has worked for more than two years without any significant problems. Most problems have been mechanical ones and have involved sticking vanes, which are easy to clean with distilled water (absolutely no oily lubricating agents).