Design Ideas: October 13, 1994
The tiny, $39 BASIC Stamp computer makes a nifty low-cost, easily programmed, RS-232C process controller (Parallax Inc, (916) 721-8217). In the circuit in Fig 1, the BASIC Stamp computer accepts commands via its RS-232C port. Its I/O pins drive a piezoelectric beeper and a demultiplexer. The demultiplexer, in turn, activates one of 16 optoisolated channels. In this application, the circuit multiplexed 16 measurement points to a precision voltmeter.
You download the program in Listing 1 (which you might have prepared on a PC) to the BASIC Stamp's nonvolatile EEPROM. Thereafter, the BASIC Stamp's program responds to a 2400-baud RS-232C control word. The control word comprises "MUX1'' followed by a space and then a function number. The number set ranges from 0 to 16 and includes 99. The number 0 turns all channels off, and a number from 1 to 16 turns on the corresponding channel. Sending 99 causes the BASIC Stamp to send its current setting back to the controlling computer.
Listing 1 and a test program, along with some documentation, are available on the EDN Readers' ftp site as /DI_SIG DI1593Z.ZIP.
| LISTING 1 |
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The piezoelectric beeper sounds whenever the BASIC Stamp computer gets an erroneous command. In agreement with the RS-232C standard, you can hang multiple BASIC Stamp computers off the same serial line, assigning each one a different name (MUX2, MUX3,...). (DI#1593]