The rate of transmission (the baud rate) is controlled by the contents of the register named SERPER . Bits 14-0 of SERPER are the baud-rate divider bits. All timing is done on the basis of a "color clock," which is 279.36ns long on NTSC machines and 281.94ns on PAL machines. If the SERPER divisor is set to the number N, then N+1 color clocks occur between samples of the state of the input pin (for receive) or between transmissions of output bits (for transmit). Thus SERPER =(3,579,545/baud)-1. On a PAL machine, SERPER =(3,546,895/baud)-1. For example, the proper SERPER value for 9600 baud on an NTSC machine is (3,579,545/9600)-1=371. With a cable of a reasonable length, the maximum reliable rate is on the order of 150,000-250,000 bits per second. Maximum rates will vary between machines. At these high rate it is not possible to handle the overhead of interrupts. The receiving end will need to be in a tight read loop. Through the use of low speed control information and high-speed bursts, a very inexpensive communication network can be built.
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