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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)
K56flex*
56,000 ~ 32,000
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
* K56flex is upgradable to ITU-T 56Kbps.
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 supports all the above
mentioned modem standards and are compatible with existing modems.
Universal Protocol Compatibility
Universal compatibility covers a broad range of ITU-T and BELL standards, and
provides data compression.
Various operation modes that can be achieved are as follows:
Standard
BPS rate
(+/-0,01%)
K56flex
56,000 - 32,000
V.34bis
33,600/31,200
V.34
28,800 - 2,400
20 Advanced Data Communications
Baud rate
Modulation
(+/-0,01%)
multiple
PCM
2,400
TCM
multiple
multiple
Carrier Freq
[Hz]
multiple
1,800
multiple
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