The image above shows a thumbnail of the interactive Java applet embedded into this page. Unfortunately, your browser is not Java-aware or Java is disabled in the browser preferences. To start the applet, please enable Java and reload this page. (You might have to restart the browser.)

Circuit Description

Just for fun, this applet shows a multiprocessor system built from three PIC16C84 microcontrollers. Each controller runs the echo.asm program for software RS-232 half-duplex data transmission (at 300 baud and 8N1) that was demonstrated in the previous applet. Because the B7 output ports are chained to the A4 input ports, each controller will echo the characters sent to it by the previous controller.

To use the demo, just type characters into the input terminal GUI window and wait for the data to ripple through the chain of microcontrollers before the data appears in the output terminal window. Again, if the window does not appear automatically, or are stacked one upon another, click on the terminal symbol to open (or close) the terminal GUI window(s) and move those windows to suitable positions on your computer screen. Because the terminal is not configured for local echo, the characters are just encoded in RS-232 format and send on the TX line of the terminal.

The program, based on Arizona Microchip application note 555, uses half-duplex communication. That means that the microcontroller cannot listen for incoming characters on port A4 while it is busy sending on port B7. Therefore, characters can be lost or garbled when transmitted too fast. Just insert a short pause after typing a character into the input terminal, to allow for the processing time required by the microcontroller. In this demo, the wait loops for the RS-232 transmission are hard-coded into the microcontroller program. You need to edit and re-assemble the program if you want to play with other baud-rates.