[NTLUG:Discuss] question about modems

[Kipton Moravec]

They don't have to stay in sync because they are async.

The start bit is what is needed to get the sync going.  In RS-232 there is 
a start bit, the data bits (7 or 8, with or without parity), and a stop bit 
for each byte sent.

More details?
O.K. here goes...

The UART chip (hardware) samples the bit stream at 8 or 16 times the bit 
rate (depending on the UART and the mode). It then uses the start bit to 
calculate where the middle of the bit is and where the middle of the rest 
of the bits should be in the data being sent. This defines where in time it 
needs to look for the next bits for the rest of the character.

The specification allows the baud rate to be off a certain percentage and 
still work. (2% or 5% can't remember and too lazy to look it up at the 
moment.)   If it is too far off and the stop bit is not where it should be 
you get a framing error.

Kip


At 12:16 PM 6/30/04, you wrote:


>This isn't about linux - but rather a general question about modems
>Once the modem handshake has been accomplished, and the modems agree on a
>frequency - how do they stay in sync once data transfer has finished?
>
>
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