Solaris Operating Environment Boot Camp - page 154


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	Solaris Operating Environment Boot Camp By David Rhodes, Dominic Butler
	Table of Contents

	Chapter 14. Connecting Serial Devices

Speed

We already mentioned that both ends of the link agree on the rate at which they will transmit data. This is measured by something known as the "baud rate," which, although not strictly correct, is commonly classed as "bits per second" (bps).

Baud and BPS

The baud rate is a measure of how many times per second a signal changes; for example, the signal on a 300-baud modem would change 300 times per second.

The number of bits transmitted per second is dependent on both the baud rate of the modem and the number of bits that it can transmit per baud. For example, if a 300-baud modem is able to transmit 1 bit per baud, it can transmit 300 bps. However, if the same modem can transmit 4 bits per baud, then the same 300-baud modem can transmit four times as many bits, or 1,200 bps.

Characters per Second

The number of characters transmitted per second (cps) depends upon the number of bits per character (bpc). For example, if the link is configured for 1 start bit, 8 data bits, no parity, and 1 stop bit, a total of 10 bits will be sent for every character, which means we can transmit 960 cps (9,600 bps/10 bpc).


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Team-Fly

	Solaris Operating Environment Boot Camp By David Rhodes, Dominic Butler
	Table of Contents

	Chapter 14. Connecting Serial Devices

Serial Devices

Two types of serial devices are available: Data Terminal Equipment and Data Communication Equipment. DTE devices are normally the source or destination devices, while DCE devices normally connect the source and destination together. Common examples of DTE equipment are terminals and printers, while the most common example of a DCE device is a modem.

The main difference between the two, as far as we are concerned , is that they transmit and receive data on different pins, as shown in Table 14.2.

Table 14.2. DTE/DCE Transmit and Receive Details

Hardware Type	Transmit	Receive
DTE	2	3
DCE	3	2

The RS-232 specification is titled "Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange," which shows it was originally designed to connect DTEs together by adding a DCE between them. It is, however, common practice to connect DTEs without the intermediate DCEwe just need to reconfigure the cable connections.

DTEDCE Connection

This is used to connect a computer to a modem. It is the designated type of connection and, as such, allows a "straight-through" cable to be easily used by connecting pin 2 (transmit) of the DTE device to pin 2 (receive) of the DCE device. A similar connection is made for transmit and receive connections on pin 3.

Figure 14.3 shows the typical cable pin-outs we would use for this type of connection.

Figure 14.3. DTEDCE (modem) cable.

graphics/14fig03.gif

DTEDTE Connection

This type of connection is used to connect a terminal or printer to a computer. For this to work, we alter some of the connections to account for the fact that both devices transmit on the same pin, and both receive on the same pin. We need to connect the transmit pin (2) on one device to the receive pin (3) on the other, and vice versain other words, cross pins 2 and 3.

This is commonly known as a "null-modem cable." Although there are a number of different "recommended" ways to make these cables, we've shown two of the more-common ones in Figure 14.4. One is often used to connect terminals, while the second is used for other DTE devices that have full modem control support.