Universal Asynchronous Receiver/transmitter - UART in Modems

UART in Modems

Modems for personal computers that plug into a motherboard slot must also include the UART function on the card. The original 8250 UART chip shipped with the IBM personal computer had no on-chip buffering for received characters, which meant that communications software performed poorly at speeds above 9600 bits/second, especially if operating under a multitasking system or if handling interrupts from disk controllers. High-speed modems used UARTs that were compatible with the original chip but which included additional FIFO buffers, giving software additional time to respond to incoming data.

A look at the performance requirements at high bit rates shows why the 16, 32, 64 or 128 byte FIFO is a necessity. The Microsoft specification for a DOS system requires that interrupts not be disabled for more than 1 millisecond at a time. Some hard disk drives and video controllers violate this specification. 9600 bit/s will deliver a character approximately every millisecond, so a 1 byte FIFO should be sufficient at this rate on a DOS system which meets the maximum interrupt disable timing. Rates above this may receive a new character before the old one has been fetched, and thus the old character will be lost. This is referred to as an overrun error and results in one or more lost characters. With error correcting modems, any lost characters will be retransmitted, but retransmission slows the connection.

A 16 byte FIFO allows up to 16 characters to be received before the computer has to service the interrupt. This increases the maximum bit rate the computer can process reliably from 9600 to 153,000 bit/s if it has a 1 millisecond interrupt dead time. A 32 byte FIFO increases the maximum rate to over 300,000 bit/s. A second benefit to having a FIFO is that the computer only has to service about 8 to 12% as many interrupts, allowing more CPU time for updating the screen, or doing other chores. Thus the computer's responses will improve as well.