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462 COMPUTER CONTROL VIA PC PRINTER PORT
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Ground FIGURE 30.1 Pinout of the DB25 parallel port connector, as used on IBM PCcompatible computers.
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TABLE 30.2 PIN PARALLEL PORT PINOUT FUNCTIONS. FUNCTION (PRINTER APPLICATION)
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Strobe Data bit 0 Data bit 1 Data bit 2 Data bit 3 Data bit 4 Data bit 5 Data bit 6 Data bit 7 Acknowledge Busy OE (out of paper, or empty) Printer online Auto line feed after carriage return Printer error Initialize printer Select/deselect printer Unused or grounded
THE FUNDAMENTAL APPROACH
0 1 2 3 Computer 4 5 6 7
0 0 0 1 0 1 1 0 8-bit word at output
FIGURE 30.2 The parallel port outputs eight bits at a time.
TABLE 30.3 BIT POSITION BIT POSITION WEIGHTS. WEIGHT
D7 D6 D5 D4 D3 D2 D1 D0
128 64 32 16 8 4 2 1
I Initialize printer I Printer interrupt
Traditionally, the status lines are the only ones that feed back into the computer (as mentioned earlier, most parallel printer ports are now bidirectional, but this is not a feature we ll get into this time around). There are five status lines, and not all parallel ports support every one. They are as follows:
I Printer error I Printer not selected I Paper error
464 COMPUTER CONTROL VIA PC PRINTER PORT
0 1 2 3 4 5 6 Parallel computer port 7 Strobe Data ready Select Printer Error Busy Inputs (from device) Data outputs
FIGURE 30.3 The minimum parallel port: eight data outputs, a STROBE (Data Ready) line, and inputs from the printer, including Select, Printer Error, and Busy. I Acknowledge I Busy
The acknowledge and busy lines are commonly used for the same thing in a printer application. However, depending on the design of the port in your computer, you can use the two separately in your own programs. (One helpful tidbit: for a printing application when the BUSY line is LOW, the ACK line is HIGH.)
Robot Experimenter s Interface
It s not generally a good idea to connect robot parts directly to a parallel port because wiring mistakes in the robot could damage the circuitry in your PC. Moreover, the parallel port in your PC may not have the drive current needed to directly operate relays, solenoids, and power transistors. By using an interface, discussed later in this chapter, you can help protect the circuitry inside your computer and provide more drive current for operating robotic control devices. This interface, called the Robot Experimenter s Interface for lack of a better name, lets your PC control up to 12 robotic functions (such as motors) and read the values of up to four robotic switches or other digital sensing devices.
ROBOT EXPERIMENTER S INTERFACE 465
TABLE 30.4 CONTROL BITS BIT
PARALLEL PORT STATUS AND CONTROL BITS.
FUNCTION
0 1 2 3 4 5 7
STATUS BITS BIT
LOW LOW LOW LOW LOW Unused
normal; HIGH normal; HIGH
output of byte of data auto linefeed after carriage return normal select printer enabled
initialize printer; HIGH deselect printer; HIGH
printer interrupt disables; HIGH
FUNCTION
0 2 3 4 5 6 7
Unused LOW LOW LOW LOW LOW printer error; HIGH no error printer on line out of paper normal
printer not on line; HIGH printer has paper; HIGH
printer acknowledges data sent; HIGH printer busy: HIGH out of paperH
Any time you mess around with a computer there is a risk of damaging it, and this goes for the circuit presented next. This is not a project you should consider if you re new to electronics and aren t sure what you re doing.
CONSTRUCTING THE INTERFACE
The schematic diagram for the Robot Experimenter s Interface is shown in Fig. 30.4. You can build it in under an hour, and it requires very few components. The interface uses a solderless experimenter s breadboard so you can create circuits right on the interface. The input and output buffering is provided by the 74367 hex buffer driver. Three such chips are used to provide 18 buffered lines, which is more than enough. You may wish to build the interface in an enclosure that is large enough to hold the breadboard and the wire-wrapping socket. Make or buy a cable using a male DB-25 connector and a four- or five-foot length of 25-connector ribbon cable. Solder the data output, status, and control line conductors to the proper pins of the 74367 ICs. Route the outputs to the bottom of the wire-wrap socket. A finished interface should look something like the one in Fig. 30.5. Using the interface requires you to provide a 5 vdc source. Do not try to power the interface from the parallel port! Use a length of 22 AWG solid
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