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Another way of increasing the number of controlled devices is to use a data demultiplexer. There are several types in both the TTL and CMOS IC families. A popular data
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474 COMPUTER CONTROL VIA PC PRINTER PORT
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demultiplexer (or demux ) is the 74154. This chip takes four binary weighted input lines (1, 2, 4, 8) and provides 16 outputs. Only one output can be on at a time. See the schematic in Fig. 30.8 to see how to hook it up. The IC is shown connected to the first four data output lines of the parallel port. You can actually connect it to any four, and you don t even have to use all four lines. With just three lines, the demux allows you to control up to eight devices. To select the device connected to the number 3 output of the demux, for example, you apply a binary 3 (0011) to its input lines. Write the line as follows:
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OUT 888, 3
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A limitation of the demux is that you can t control more than one device connected to it at any one time. You can t, for example, attach both drive motors to the demux outputs and have
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24 Vcc 0 19 18 Enable Data Input 3 Pin 2 Pin 3 Inputs Pin 4 Pin 5 21 20 C D 23 22 4 A B 74154 5 6 7 8 1 2 1 2 3 4 5 6 7 8 9
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FIGURE 30.8 Basic wiring diagram for the 74154 demultiplexer chip.
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INPUTTING DATA 475
them on at the same time. There will be many times, however, when your robot will only be doing one thing (such as triggering an ultrasonic ranger). In these cases, the demux is perfect.
EXTERNAL ADDRESSING
As mentioned earlier in this chapter, all sorts of data and control lines are inside the computer, on the microprocessor bus. There is also a set of special-purpose lines, the address lines, that are used to pass data to specific devices and expansion boards. For example, you address the data output lines of the parallel port by sending out the address 888. The address for the parallel port triggers just the parallel port, but with some ingenuity (and no extra components) you can wire up a subaddress scheme so the one parallel port can fully control a very large number of devices. This is the third and most sophisticated way to sap all the power out of the parallel port. You can disable the 74367 hex buffer IC, which is used to link the port to the outside world. In the Robot Experimenter s Interface, the ENABLE lines of the chip, pins 1 and 15, are held LOW by tying them to the ground, so data is passed from the input to the output. When the ENABLE pins are brought HIGH, the outputs are driven to a high-impedance state and no longer pass digital data. In this way, the 74367 acts as a kind of valve. The two ENABLE lines control different input/output pairs, as shown in Fig. 30.9. The high-impedance disabled state is engineered so that many 74367 chips can be paralleled on the same data lines, without loading the rest of the circuit. You can use the ENABLE pins of the 74367 and a few of the unused control lines in the parallel port to make yourself an electronic data selector switch. In operation, you output a binary word onto the data output lines. You then send the word to the desired device by addressing it with the control lines. Here is an example: Let s say that you have connected three subaddress ports to the parallel printer port, as shown in Fig. 30.10. Control lines 1, 2, and 3 are connected to the ENABLE pins of the 74367. The inputs of the three 74367s are connected together. The outputs of each feed to the specific device. To turn on bits 0 and 1 on device 2, enter the following lines into Basic and run the program:
OUT 888, 3 OUT 890, 2
The first line of the program outputs a decimal 3 to the data output register. That places the binary bit pattern 00000011 on the parallel port data output lines. The next line enables device 2 because it turns on the second 74367.
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