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Chapter 4 - Advanced Data Communications
4-2
LED
V34 V32B Indicates Status
OFF OFF Modem is in V.22bis, V.22, or mode slower than 9600bps.
OFF ON Modem is in V.32bis or V.32 mode.
ON OFF Modem is in V.34 mode.
ON ON Modem is in a ZyXEL proprietary mode
(ZyX 19.2, ZyX 16.8, or ZyCellular).
DTE Interface
DTE and DCE are terms used in data communication. DTE stands for Data
Terminal Equipment and DCE stands for Data Communication Equipment. The
computer or terminal is the DTE and the modem is the DCE. The DTE interface
used by the Omni 288S is an RS-232 with throughput speeds up to 460.8 Kbps.
Synchronous and Asynchronous Communications
There are two kinds of serial data communication. One is called synchronous, and
the other is called asynchronous. In synchronous communication, data is
transmitted and received bit by bit and is timed by an accompanying clock signal.
In asynchronous communication, data is sent character by character (or octet by
octet), and the idle time between characters is variable. Since no clock signal is
sent, character timing is recovered from the data itself.
A PC's COM1 and COM2 are asynchronous serial ports. Most PCs' and UNIX
systems' serial data communications are asynchronous. The serial data
communication on an IBM mainframe or mini is synchronous.
UART
A UART (Universal Asynchronous Receiver Transmitter) is the device used in a
DTE or DCE for asynchronous data reception and transmission. The standard
UART device used in PCs is of the NS16450 type. For high-speed serial data
transfers (38400 bps and up), the PC may not serve the UART fast enough and
data may be lost. In this case, a UART with data buffer is needed, such as the
NS16550A type device.
Serial Port
A serial port is the serial data connector together with its internal circuit on the
DTE or DCE with electrical and mechanical characteristics according to RS-
232C. Since some signals travel from the DTE port to DCE port, and some
signals travel in the opposite direction, the signal pin is a transmitter on one port
and a receiver on the other. The DTE serial port is different from the DCE serial
port in terms of signals on the connector pins. There are also mechanical
differences in terms of male (with pins) or female (with holes) connectors.