ZyXEL Communications COMET 336, User Manual

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Advanced Data Communications 23
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.
Serial RS-232C Cable
A serial RS-232C cable is used to connect a DTE port to a DCE port. Do not use a null-
modem cable (which may be used to connect two DTEs directly with each other through
their serial ports). A normal RS-232C connector has 25 pins, and a normal RS-232C
cable has 25 wires. Many signals in the RS-232C are not used in common applications,
and a 9-wire RS232C cable is sufficient in most applications. The PCAT's serial port
has only 9 connector pins, thus eliminating unnecessary pins. For high-speed DTE-DCE
communication, use a low-capacitance cable (as short as possible).
Communication Protocols and Speeds
The most commonly used ITU-T modem standards for 2-wire dial-up lines are
summarized in the chart below:
Standard Speed (bps)
V.34bis 33,600 / 31,200
V.34 28,800 ~ 2,400
V.32bis 14,400 / 12,000 / 7,200
V.32 9,600 / 4,800
V.22bis* 2,400 / 1,200
V.22 1,200
V.21 300
V.23 1,200
bis is the old French word for second.
In the USA, Bell Systems used to create de facto standards such as Bell 212A for 1200
bps modems and Bell 103 for 300 bps modems. The Comet 336 supports all the above
mentioned modem standards and are compatible with existing modems.