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Stopbit Command Question

So I am running through some configs that one of our previous engineers used in a router config and came across the stopbit 1 command on line con 0.  Can someone explain what the stopbit is and why you would use this command at all?  I have just never seen it used before and cant seem to find documentation stating as to why you would use it.  Thanks for any help.

1 ACCEPTED SOLUTION

Accepted Solutions
Richard Burts
Hall of Fame Guru

The console is set for async communication and the stop bits are used for controlling the stream of bits. I believe that the default was to use 2 stop bits and for some connections or some emulators you could specify use of a single one. If you look in your favorite terminal emulator for the setting for serial interfaces you will find that stop bits is one of the fields. Here is a link that will have more details if you want them

https://www.cisco.com/c/en/us/td/docs/ios/12_2/configfun/command/reference/ffun_r/frf003.html

 

HTH

 

Rick

HTH

Rick

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6 REPLIES 6
Richard Burts
Hall of Fame Guru

The console is set for async communication and the stop bits are used for controlling the stream of bits. I believe that the default was to use 2 stop bits and for some connections or some emulators you could specify use of a single one. If you look in your favorite terminal emulator for the setting for serial interfaces you will find that stop bits is one of the fields. Here is a link that will have more details if you want them

https://www.cisco.com/c/en/us/td/docs/ios/12_2/configfun/command/reference/ffun_r/frf003.html

 

HTH

 

Rick

HTH

Rick
Giuseppe Larosa
Hall of Fame Master

Hello Brandon,

a console connection is actually an asynchronous serial line.

There is no clock and instead of sending whole frames over it we send a character per time.

 

The stopbit(s) is a feature that provides and additional signal over the wire to indicate the end of the sent character.

 simply the previous engineer tried to use it and the he/she forgot to remove.

 

 

Edit:

as Rick has noted checking the command reference the default is to use two stop bits so we send an 'a' as 8+2 10 bits over the wire. 8 bits are the ASCII code for 'a' and then there are two stop bits.

As I have written above it is a protection mechanism to detect errors on the async communications.

 

Hope to help

Giuseppe

 

 

 

chrihussey
Collaborator

From Wiki leaks (https://en.wikipedia.org/wiki/Serial_port)

Stop bits sent at the end of every character allow the receiving signal hardware to detect the end of a character and to resynchronise with the character stream.

So if you're typing on the console port, after each character a 1, 1.5 or 2 bits are sent to keep communications in synch.

I checked a few Cisco devices and the default appears to be 2. It may vary with some platforms. The terminal emulator I've used for years is set to 1, yet I've never had an issue. So that might be an indication of how sensitive this setting is to communication.

So, your predecessor may have been trying to figure something out and it may have helped, maybe not.

I prefer to keep the defaults, so if you make it 2 and can still communicate on the console port I'd keep it that way.

Hope this helps

 

 

Thank you all for the prompt responses and the thorough answers.  That gave me the clarity I needed.  Thanks again.  

Joseph W. Doherty
Hall of Fame Expert

As the others have noted, the console port is async, and async uses a start and stop "bit" to "frame" a byte. These start/stop "bits" are how the sender and receiver keep in sync for where characters begin and end.

The stop bit, as Chris notes, can be set to 1, 1.5 or 2 bits. If you're wondering how you can have 1.5 bits, the stop "bit" is really holding a signal state on the wire for a certain amount of bit time at the current transmission rate. So, it's possible to have a stop "bit" be 1.5 bit time, as it's actually not a data bit.

As to why the stop bit was set to 1, that would be to slightly increase transmission speed. I.e. rather than setting a character using 11 bits' time, it sends the same character in 10 bits's time or 10/11 the same time, or 11/10 (10%) times faster.

mpofut0901
Beginner

The stopbit 1 command on line con 0 is used to match the setting on your terminal emulation software (Like HyperTerminal or Putty) so that the characters are displayed properly.