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One communications system is the parallel port. Though something of a legacy device, having been mostly replaced by USB, most computers still have a parallel port on them. When several activies are performed at the same time, they are said to occur in parallel. Thus, parallel programming involves two or more programs executing at the same time, and parallel communication involves multiple bits of data being sent at the same time. The basic, plain-vanilla IBM PC parallel port is laid out as shown in Fig. 15-1. Note that there are two connectors displayed in this gure. The one on the left is normally found on the computer and is a female connector. The matching plug is, for hopefully obvious reasons, a male connector. I think that engineers probably spend far too much time in the company of other engineers and they really should get out more. You communicate to the parallel port through three 8-bit registers. These registers are locations in the computer s memory that connect to the port. Writing a value to the register sends information to the port, and reading the register returns data from the port. The data register sends eight bits of data at a time from the computer to the peripheral. The control register has four more bits that are used to change the behavior of the device. Finally, the status register lets the device send status information back to the computer. Printers are the most
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CHAPTER 15 Communication
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Fig. 15-1.
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common peripheral for the parallel port, and the port de nitions re ect this (Table 15-1). Note that a bar over a signal indicates that the bit is inverted. If you write a 1 to the bit, it comes out as an electrical zero. Likewise, a 0 comes out as a positive voltage. The reverse is also true, a zero input to the port reads as a binary 1 to the program. The precise meaning of these signals and how to use them can be found in the reference named below. A signi cant failing of the standard parallel port is that it can not transfer data e ciently from the device back to the controlling computer. Other parallel standards address this, however. For more details on parallel ports, look to Jan Axelson s book Parallel Port Complete (Pub Resource, 1997).
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While the parallel port sends eight bits simultaneously, the serial port transmits bits one at a time. Where the parallel port is an eight-lane highway, the serial port is a one-lane backwoods road. In spite of being comparatively wimpy, the serial port is used more often than the parallel port. Part of its attraction may be that it uses fewer wires than the parallel port. Though the serial port de nes nine pins in its interface
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Communication
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Table 15-1 Parallel port pins DB25 pin 2 3 4 5 6 7 8 9
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Signal name Data0 Data1 Data2 Data3 Data4 Data5 Data6 Data7
Error Select Paper End Acknowledge Busy
15 13 12 10 11
Strobe AutoLF Init SelectIn
1 14 16 17
CHAPTER 15 Communication
Fig. 15-2.
Serial port
Table 15-2
Serial port pins DB9 pin 1 2 3 4 5 6 7 8 9
Signal name CD RX TX DTR GND DSR RTS CTS RI
(Fig. 15-2), you can communicate using just three of them, GND, RX, and TX (Table 15-2). The serial port is also bidirectional, in that it sends and receives data equally well. Since serial ports are ubiquitous, almost all microcontrollers can communicate using the serial protocol. You can also buy sensors that send their information using serial communication. This discussion of serial communication centers on the port found on most PCs, the RS232 serial port using a DB9 connector. There are actually two layers involved in using a serial port. There is the physical connection which involves the connector and the voltages applied to it, and there is the way these voltages are organized to transmit information.
Communication
The RS232 electrical structure can communicate over wires of at most 50 feet long. A di erent electrical system, RS485, can be used to send these serial signals over wires nearly a mile long. RS485 is an especially useful data carrier inside robots, since robots are electrically very noisy and the RS485 system is especially resistant to this noise. There are also other specialized serial communications protocols. Your car probably uses the CAN serial interface, and in some systems the integrated circuits communicate using I2C. There are other serial protocols, many others in fact, but they all send and receive data one bit at a time. More information can be found in Jan Axelson s Serial Port Complete (Independent Publishers Group, 1998), as well as Edwin Wise s Applied Robotics (Delmar Learning, 1999) and Applied Robotics II (Prompt, 2002).
The Universal Serial Bus, or USB, is a newer communications protocol that is beginning to edge out both the parallel and serial ports. It is more complicated than the serial port, but it supports all manner of peripherals, from computer mice to printers. As time marches by, more microcontrollers are supporting USB so we should be seeing it in more robots in the future. Needless to say, Jan Axelson has a book on the subject, USB Complete (Lakeview Research, 2nd ed., 2001).
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