Page 307 RS 232 terminals are gradually dying off being replaced by PCs but

Page 307 rs 232 terminals are gradually dying off

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[Page 307] RS-232 terminals are gradually dying off, being replaced by PCs, but they are still encountered on older mainframe systems, especially in banking, airline reservation, and similar applications. Terminal programs that allow a remote computer to simulate a terminal are still widely used, however. To print a character, the terminal driver writes the character to the interface card, where it is buffered and then shifted out over the serial line one bit at a time by the UART. Even
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at 56,000 bps, it takes just over 140 microsec to send a character. As a result of this slow transmission rate, the driver generally outputs a character to the RS-232 card and blocks, waiting for the interrupt generated by the interface when the character has been transmitted and the UART is able to accept another character. The UART can simultaneously send and receive characters, as its name implies. An interrupt is also generated when a character is received, and usually a small number of input characters can be buffered. The terminal driver must check a register when an interrupt is received to determine the cause of the interrupt. Some interface cards have a CPU and memory and can handle multiple lines, taking over much of the I/O load from the main CPU. RS-232 terminals can be subdivided into categories, as mentioned above. The simplest ones were hardcopy (printing) terminals. Characters typed on the keyboard were transmitted to the computer. Characters sent by the computer were typed on the paper. These terminals are obsolete and rarely seen any more. Dumb CRT terminals work the same way, only with a screen instead of paper. These are frequently called "glass ttys" because they are functionally the same as hardcopy ttys. (The term "tty" is an abbreviation for Teletype, ® a former company that pioneered in the computer terminal business; "tty" has come to mean any terminal.) Glass ttys are also obsolete. Intelligent CRT terminals are in fact miniature, specialized computers. They have a CPU and memory and contain software, usually in ROM. From the operating system's viewpoint, the main difference between a glass tty and an intelligent terminal is that the latter understands certain escape sequences. For example, by sending the ASCII ESC character (033), followed by various other characters, it may be possible to move the cursor to any position on the screen, insert text in the middle of the screen, and so forth. 3.8.2. Terminal Software The keyboard and display are almost independent devices, so we will treat them separately here. (They are not quite independent, since typed characters must be displayed on the screen.) In MINIX 3 the keyboard and screen drivers are part of the same process; in other systems they may be split into distinct drivers. [Page 308] Input Software The basic job of the keyboard driver is to collect input from the keyboard and pass it to user programs when they read from the terminal. Two possible philosophies can be adopted for the driver. In the first one, the driver's job is just to accept input and pass it upward unmodified. A program reading from the terminal gets a raw sequence of ASCII
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