The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 12
Listing 12-2. Reads received data at the decoder.
10 REM 8255 mode set: Ports A,C = output; Port B = input
20 XBY(0FC03H)=82H
30 REM A=decoder address (0-15)
40 A=2
50 REM Write address to Port C, turn TE (PC.7) off (high)
60 XBY(0FC02H)=80H+A.OR.XBY(0FC02H)
70 REM Use edge-triggered interrupt
80 TCON=TCON.OR.4
90 DO
100 REM Wait for interrupt
110 ONEX1 500
120 WHILE 1=1
130 END
480 REM On interrupt 1, print received data
490 REM Data is at Port B, bits 4-7
500 PH0.(XBY(0FC01H).AND.0F0H)/10H
510 RETI
You can have more than one receiver in a link. If each has a unique address, it will accept
only the transmissions meant for it.
Increasing the Distance
When you have your link up and running, one of the first challenges is to see how far you
can reliably transmit. Two ways to increase the length of the link are by increasing the power
of the transmitted signal, and by focusing the signal more precisely on the receiver.
About Infrared Energy
But first, some basics about infrared. Like visible light, infrared energy is a form of
electromagnetic radiation. Infra means below, and infrared frequencies are just below those
of red light. Infrared frequencies are invisible, or beyond the range detected by the human
eye. Since wavelength is the inverse of frequency, infrared wavelengths are longer than those
of visible light. Visible light covers the range 400-700 nm (nanometers), while infrared
includes 700 nm through 1 million nm. (400 nanometers is 0.4 micron, or 4000 angstroms.)
Infrared-emitting diodes, or IREDs, are low-cost, widely available sources of infrared
energy. An IRED is a semiconductor diode that emits infrared energy when a forward current
passes through it, much as an LED emits visible light.
212 The Microcontroller Idea Book