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What is a FIFO buffer? How does it work?

FIFO stands for "First In/First Out" and is a way for the UART to process data more smoothly. It is a memory device that allows for flow control from the modem to the CPU and vice versa. The UART stores incoming data in the FIFO buffer, and the FIFO buffer holds it until the CPU is ready for it. In the other direction, the CPU can send a bunch of data to the modem, which sits in the FIFO buffer and streams out as the UART is ready to pass it to the modem. There are actually two separate buffers, one for incoming and one for outgoing.

FIFO buffers can be adjusted for the number of bytes they can hold at a time. Knocking the FIFO buffers down a notch can help with connection problems and disconnections, because without them, the modems respond more directly to each other. But as the FIFO buffers are turned down, performance will be decreased. Adjusting the FIFOs is generally a bad idea and won't help as much as they hurt. If this is the problem, it points to more serious hardware issues. Basically, don't bother turning down the FIFO buffers at all, unless you see "CRC Overrun Errors". (This is what happens when you put a 56K modem on an old machine with 16450 UARTs).


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