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MODEM COMMUNICATION TO ABB RELAYS
Another new technology used in modems is one called, Trellis Encoding Technology.
One of the
modems presented in this paper uses this technology which evaluates speed optimization and fast
forward error detection/correction technology. Within the present V. standards, error detection/correction
and line speed balancing improves with each technology. One modem shall be used in this paper which
uses Trellis Encoding Technology.
THE TRICKY THING ABOUT BAUD RATES
Baud rate is defined as the amount of changes a signal can undergo in 1 second. With FSK modems in
the initial days of the Bell 202 modem, 1 baud = 1 bit. Today, with the complexity of modem technology,
one bit does not equal one baud.
As illustrated in the descriptions of DPSK and QAM, one transition of
signal may not equal one baud in that two bits may represent 4 combinations, 3 bits may represent 8
combinations, 4 bits may represent 16 combinations, 8 bits may represent 256 combinations, and 12 bits
may represent 4,096 combinations. Thus operation over a standard frequency 300, 600, or 2400 hertz
(audio) may yield ( when signals are decoded into digital signals) baud rates of up to 33, 600 bits per
second.
Standards
Early modems were defined as per their operating baud rate.
An international committee the ITU-T
(International Telecommunications Union) developed standards defining the operation of modems.
Today, the V. (VEE DOT) standard is recognized as the modem definition standard to which modems are
designed. Some standards are listed below:
V.29 BIS - 2,400 Baud : 9,600 Bits per second: 2 Wire Full Duplex, 4-DSPK, 16 QAM
V.32 BIS - 2,400 Baud: 14,400 Bits per second: 2 Wire Full Duplex 64- QAM,
V.34 +
- 2,400 Baud: 33,600 Bits per second: 2 Wire Full Duplex 4,096 QAM.
With the increasing complexity of modem technology, another innovation came about increasing the
acceptance of telephone modem technology in circuits, Hayes AT command set. Hayes was one of the
pre-eminent manufacturers of modem technology in the early 70's. They developed a command set
which allowed a modem to be placed in several operational modes. Modems could be configured "on the
fly" to auto-answer, change transmission/reception speeds, enable data encoding modes, dial out phone
numbers ..... as well as other capabilities.
With the introduction of the Hayes AT command set, integration of modems into more common
acceptance within a variety of applications.
Configuration could occur using a commonly supplied
"WINDOWS" Terminal Emulator program.
When the terminal connected with the modem the "AT"
command could be sent to the modem with the appropriate command. Unfortunately over time there has
been a deviation in the HAYES command set in that there is no such thing as a "STANDARD HAYES AT
COMMAND" set.
10 Bit Versus 11 Bit Modems
Commercially available dial-up modems, such as those generally sold through chain electronic stores
may be used with many of the protocols offered in the ABB Protective relays. Modems such as those
allowing telephone connectivity using 10 bit protocols are generally those available inexpensively. A 10
bit telephone modem is in the cost area of $100 (120 VAC operation) whereas 11 bit modems are in the
area of cost of $1500 (120 VAC operation) [COSTS QUOTED ARE FOR YEAR 2000]. Modems using
125 VDC power sources are much more expensive than those quoted for 120 VAC operation.
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