and parity bit in each word contain an even number of 1s. With odd parity, the data bits and parity bit in each word contain an odd number of 1s. For example, assume the data bits to send are 0000001^. With even parity, the parity bit is 1 to bring the total number of 1s in the data and parity bits to an even number. With odd parity, the parity bit is 0 to keep the total number of 1s odd. If the data bits are 0000011 b, with even parity, the parity bit is 0, and with odd parity, the parity bit is 1. If the communications are using 7-E-1 format, a receiving computer that receives a byte with an odd number of 1s knows the data didn’t transmit correctly. Mark and space parity are forms of stick parity. With mark parity, the parity bit is always 1, and with space parity, the parity bit is always zero. One use for these parities is in 9-bit networks that use mark and space bits to indicate whether the data bits contain an address or data. Chapter 12 has more about 9-bit networks. Most UARTs support 7- and 8-bit data. Some UARTs support anywhere from 5 to 8 data bits. The data bits in a transmitted word are sometimes referred to as a character and may in fact represent a text character. 12 Formats and Protocols A SYNCHRONOUS TRANSMISSION ■TRANSMITTER SENDS BITS ON CLOCK'S FALLING EDGE RECEIVER READS BITS ON CLOCK'S RISING EDGE \/ М/ 171 CLOCK DATA 61 H BIT 7 ■ 0 1 BIT 0 1 0 0 0 0 MANY SYNCHRONOUS PROTOCOLS SEND MSB FIRST B ASYNCHRONOUS TRANSMISSION TRANSMITTER USES AN INTERNAL CLOCK TO DETERMINE WHEN TO SEND EACH BIT. r RECEIVER DETECTS THE FALLING EDGE OF START, THEN USES ITS INTERNAL CLOCK TO READ THE „ FOLLOWING BITS NEAR THEIR CENTERS. DATA 61 H ■ START BIT 0 1 BIT 1 0 ; 0 ; 0 ; 0 ■ ■ BIT 7 STOP BIT ; l ; l ; 0 ASYNCHRONOUS PROTOCOLS SEND LSB FIRST Figure 2-1: Synchronous transmissions include a clock line, while asynchronous transmissions require each computer to have its own clock. For receivers that require a little extra time to ac prom-electric.ru