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Micro House PC Hardware Library Volume II: Network Interface Cards And Modems
by Micro House International, Inc. and Scott Mueller
Que, Macmillan Computer Publishing
ISBN: 078971664x Pub Date: 06/17/98
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Chapter 1
Communications and Networking
Most computer-to-computer connections occur through a serial port or a network adapter. In this chapter, you explore ways to connect your PC to other computers. Such connections enable you to transfer and share files, send electronic mail, access software on other computers, and generally make two or more computers behave as a team.
Note:
For a much more detailed account of how to upgrade and repair networks, refer to Que’s Upgrading and Repairing Networks, ISBN 0-7897-0181-2.
Using Communications Ports and Devices
The basic communications ports in any PC system are the serial and parallel ports. The serial ports are used primarily for devices that must communicate bidirectionally with the system. Such devices include modems, mice, scanners, digitizers, and any other devices that “talk to” and receive information from the PC.
Serial Ports
The asynchronous serial interface is the primary system-to-system communications port. Asynchronous means that no synchronization or clocking signal is present, so characters may be sent with any arbitrary time spacing.
Each character sent over a serial connection is framed by a standard start and stop signal. A single 0 bit, called the start bit, precedes each character to tell the receiving system that the next 8 bits constitute a byte of data. One or two stop bits follow the character to signal that the character has been sent. At the receiving end of the communication, characters are recognized by the start and stop signals instead of by the timing of their arrival. The asynchronous interface is character-oriented and has about a 20 percent overhead for the extra information needed to identify each character.
Serial refers to data sent over a single wire, with each bit lining up in a series as the bits are sent. This type of communication is used over the phone system, because this system provides one wire for data in each direction. Add-on serial ports for the PC are available from many manufacturers. You usually can find these ports on one of the multifunction boards available or on a board with at least a parallel port. Figure 1-1 shows the standard 9-pin AT-style serial port, and Figure 1-2 shows the 25-pin version.
FIG. 1-1 AT-style 9-pin serial-port connector specifications.
FIG. 1-2 Standard 25-pin serial-port connector specifications.
Serial ports may connect to a variety of devices such as modems, plotters, printers, other computers, bar code readers, scales, and device control circuits. Basically, anything that needs a two-way connection to the PC uses the industry-standard Reference Standard number 232 revision c (RS-232c) serial port. This device enables data transfer between otherwise incompatible devices. Tables 1.1, 1.2, and 1.3 show the pinouts of the 9-pin (AT-style), 25-pin, and 9-pin-to-25-pin serial connectors.
Table 1.1 9-Pin (AT) Serial Port Connector.
|
| Pin
| Signal
| Description
| I/O
|
|
| 1
| CD
| Carrier detect
| In
|
| 2
| RD
| Receive data
| In
|
| 3
| TD
| Transmit data
| Out
|
| 4
| DTR
| Data terminal ready
| Out
|
| 5
| SG
| Signal ground
| —
|
| 6
| DSR
| Data set ready
| In
|
| 7
| RTS
| Request to send
| Out
|
| 8
| CTS
| Clear to send
| In
|
| 9
| RI
| Ring indicator
| In
|
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Table 1.2 25-Pin (PC, XT, and PS/2) Serial Port Connector.
|
| Pin
| Signal
| Description
| I/O
|
|
| 1
| —
| Chassis ground
| —
|
| 2
| TD
| Transmit data
| Out
|
| 3
| RD
| Receive data
| In
|
| 4
| RTS
| Request to send
| Out
|
| 5
| CTS
| Clear to send
| In
|
| 6
| DSR
| Data set ready
| In
|
| 7
| SG
| Signal ground
| —
|
| 8
| CD
| Carrier detect
| In
|
| 9
| —
| +Transmit current loop return
| Out
|
| 11
| —
| -Transmit current loop data
| Out
|
| 18
| —
| +Receive current loop data
| In
|
| 20
| DTR
| Data terminal ready
| Out
|
| 22
| RI
| Ring indicator
| In
|
| 25
| —
| -Receive current loop return
| In
|
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Table 1.3 9-Pin to 25-Pin Serial Cable Adapter Connections.
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| 9-Pin
| 25-Pin
| Signal
| Description
|
|
| 1
| 8
| CD
| Carrier detect
|
| 2
| 3
| RD
| Receive data
|
| 3
| 2
| TD
| Transmit data
|
| 4
| 20
| DTR
| Data terminal ready
|
| 5
| 7
| SG
| Signal ground
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| 6
| 6
| DSR
| Data set ready
|
| 7
| 4
| RTS
| Request to send
|
| 8
| 5
| CTS
| Clear to send
|
| 9
| 22
| RI
| Ring indicator
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Note:
Macintosh systems use a similar serial interface defined as RS-422. Most external modems in use today can interface with either RS-232 or RS-422, but it is safest to make sure that the external modem you get for your PC is designed for a PC, not a Macintosh.
The heart of any serial port is the Universal Asynchronous Receiver/Transmitter (UART) chip. This chip completely controls the process of breaking the native parallel data within the PC into serial format, and later converting serial data back into the parallel format.
There are several types of UART chips on the market. The original PC and XT used the 8250 UART, which is still used in many low-price serial cards on the market. In the PC/AT (or other systems based on at least an 80286 processor), the 16450 UART is used. The only difference between these chips is their suitability for high-speed communications. The 16450 is better suited for high-speed communications than the 8250; otherwise, both chips appear identical to most software.
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