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Appendix I, Terminals and Terminal Emulators


VT Terminals

Serial communications

Serial Modems


PC Terminals

X Terminals




Serial Modems

Before ISP's and Internet Cafes, remote access meant using a serial modem to dial another modem connected to the serial port on a computer. Initially, there were no PC's involved. A terminal would be connected to the modem with a serial cable and the modem would be connected to its counterpart with the public telephone lines or a leased line. The serial speeds which early modems were able to achieve were underwhelming. Typically they were 4 to 16 times slower than what the user experienced at the computer site. The first modems lacked error control or correction making them less usable on bad telephone lines. And to make matters worse, they were expensive, anywhere from $500 to $2,000.

But the world was not going to be forced to stay in its office and the demand for modems drove the industry to produce higher quality modems for less money. Bit by bit, (no pun intended,) all the shortcomings were eliminated. New modem communication protocols allowed the serial speed to be increased to that used in the office and made remote work on terminals more efficient. The new protocols provided error checking which allowed modems to be used on bad telephone lines without the usual and frequent interruption due to disconnection. And the $2,000 modem was reduced in cost to less than a hundred dollars.

The serial speed of modem connections is determined by the modem communication protocols supported by the modem and the quality of the telephone line. The modem communication protocol is the software that the modems use to talk to each other. The software is written and saved on a chip in the modem so it would be more accurately called, firmware. The better the software, the faster the modem can transmit and receive data. The electronic hardware components must also be good to transmit and receive data fast but the entire package must be matched so the package is represented as a modem protocol. The protocols typically have names like V.32, V.42, V.90.

Now days modems are comparatively cheap and it would be a waste of energy to use less than a V.32 protocol modem, at 9600 baud for serial terminal use. Indeed, any less speedy modem is probably not in use today. The much faster, V.90 modem protocol, is supported on 100% of modems sold today. With data transfer rates of up to 56K, it more than adequately supports serial terminal communications and is more commonly used for network communications.

Typically, modems are downward compatible in protocol so if you have a V.90 modem you will have no problem connecting to any other modem even if it is old and doesn't support the V.90 protocol. And as long as both of the modems are V.32 or better, a serial connection can be made at a baud rate sufficient for serial terminal use. Similarly, if you are forced to use an older modem, as long as it supports, at least the V.32 modem protocol, it will be fast enough for fast serial communication and it will also connection to higher speed modems which will be downward compatible.

© 2000 - 2008 -- CCSS, William A. Pedersen

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