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Why can serial communication have upto 2 stop bits? Is one not enough?

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Reading about serial port communication shows that while usually there is only 1 stop bit and 1 start bit, there can be upto 2 stop bits. When/Why would one need 2 stop bits?
 

hello,

when using 7 bits data ...on old RS232 equipment ..like teletype
2 stop bits to keep a 10 moments trame
1 start + 7 data +( no parity) + 2 stop = 10 bits

1 start + 7 data + parity +1 stop => 10
1 start + 8 data + no parity+1 stop=> 10
 

One reason relates to OLD mechanical teleprinters where there was a requirement for a minimum delay between characters so the wholly mechanical receiver had time to print the character (and possibly actually do the physical carriage return or line feed) before the next character started into the mechanics.

These things used a solenoid, a motor and clever mechanics to implement the serial line receiver and transmitter, no electronics to speak of, line speeds were typically 45 Baud or so.

Regards, Dan.
 

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