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Recall that a parallel port has a third and final input register for providing status. Most parallel ports support four or five status lines, which you can use to input data back into the computer. The Robot Experimenter s Interface uses the four status lines you are likely to find in any parallel port. To read data from the port, you use the Basic INP statement (INP for input). The input command is used as follows:
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10 I
8 GND 74367
Inputs OE L L H I L H X
Outputs O L H HI-Z
Truth table -- 74367 Y=INP(x)
FIGURE 30.9 The internal configuration of the 74367 chip. Note the two independent ENABLE lines, on pins 1 and 15.
In place of x you put the decimal address of the port you want to read. In the case of the main printer port at starting address 888, the address of the status register is 889. The Y is a variable used to store the return value for future use in the program. For testing, you can PRINT the value of Y, which shows the decimal equivalent of the binary bit pattern on the screen. Listing 30.3 is a sample program that displays the current values of the four inputs connected to the Robot Experimenter s Interface. The values are shown as 0 ( false ) and -1 ( true ). Bear in mind that the Busy and Online lines are active-low; therefore their logic is the reverse of the others. The code in the test program compensates for the active-low condition by reversing the logic in the If expressions. Also note the less-than-straightforward method for determining if pins 15 and 12 are triggered exclusively. These extra If tests are needed because the parallel port (most, anyway) will automatically bring pin 12 HIGH if pin 15 is brought HIGH. Weirdness is also
INPUTTING DATA 477
Parallel port connector ENABLE 74367
Data lines
Data outputs
Data lines Pin 17 Pins 1-9 D0-D7 Data lines 74367 74367 ENABLE
Data outputs
Data outputs
Pin 14
ENABLE
FIGURE 30.10 Block diagram for a selectable parallel port, using three 74367 ICs to independently control three separate devices.
encountered if pin 15 is brought HIGH while trying to read the values of pins 10 and 11. The port reads pins 10 and 11 as LOW, even though they may be HIGH on the interface. Again, this is the action of pin 15 (printer error), and for this reason, it s usually a good idea to limit its use or to ensure that the values of other inputs are ignored whenever pin 15 is HIGH.
LISTING 30.3.
DIM BaseAddress AS INTEGER, StatusPort AS INTEGER DIM DataPort AS INTEGER, ControlPort AS INTEGER DIM x AS INTEGER, Count AS INTEGER BaseAddress = 888 DataPort = BaseAddress StatusPort = BaseAddress + 1 ControlPort = BaseAddress + 2 WHILE (1) x = INP(StatusPort) + 1 IF (x AND 64) = 64 THEN PRINT "Pin 10: 1" ELSE PRINT "Pin 10: 0" END IF IF (x AND 128) <> 128 THEN PRINT "Pin 11: 1" ELSE
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PRINT "Pin 11: 0" END IF IF ((x AND 16) = 0) AND ((x AND 8) = 0) THEN PRINT "Pin 12: 1" ELSE PRINT "Pin 12: 0" END IF IF ((x AND 32) = 32) AND (x AND 8) = 8 AND (x AND 16) = 16 THEN PRINT "Pin 15: 1" ELSE IF ((x AND 32) = 0) AND (x AND 8) = 0 THEN PRINT "Pin 15: 1" ELSE PRINT "Pin 15: 0" END IF END IF PRINT "": PRINT "" FOR Count = 1 TO 10000: NEXT Count CLS WEND
Before moving on, notice the use of the DIM keyword in the program shown in Listing 30.3. The DIM (for dimension ) keyword tells Basic what kind of variables are used in the program. While using DIM is not absolutely mandatory (in QBasic and later), you ll find that adopting it in your programs will not only help reduce errors and bugs. Most of all, it will make your programs run much faster. Without the DIM keyword, the Basic interpreter creates an all-purpose variant variable type that can hold numbers of different sizes, as well as strings. Every use of the variable requires Basic to rethink the best way to store the variable contents, and this takes time.
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