Cisco 2500 Series Routers

Field Notice: Terminal Server Break Character on Cisco Access Servers

Updated August 18, 2000

April 21, 1998

Products Affected

This field notice affects the following products:

  • AS2509 & 2509ET

  • AS2511 & 2511ET

  • AS2509-RJ

  • AS2511-RJ

Problem Description

Some devices connected to an asynchronous port may be put into "maintenance" mode if the Access Server loses power.

When an Access Server is used as a "console server" to allow networked access to console port of UNIX servers or workstations, certain machines will "halt" and stop running when the access server is power cycled or when their console cables are connected and disconnected.

Most frequently, this problem is seem with UNIX systems from Sun Microsystems.

Once the systems have halted, you can attach to each console and restore the service by typing "c" for continue, but the downtime is unacceptable.


A BREAK is defined as a "space" condition on an RS-232/V.24 line for some number of milliseconds (typically about 125ms to 500ms). The normal condition on the line is a "mark".

A space = logical 0 = positive voltage between +3 and +12V on RS-232/V.24

A mark = logical 1 = negative voltage between -3 and -12V on RS-232/V.24

A normal async character starts with:

  • a start bit (space)

  • 7 or 8 data bits (marks or spaces)

  • an optional parity bit

  • 1, 1.5 or 2 stop bits (mark)

The BREAK sequence starts of with a start bit (space), followed by a number of spaces (all zeros) for an amount of time greater then the transmission of a "normal" asynchronous character; therefore, the receiving side knows/detects it as a BREAK condition requiring attention.

When attaching or removing a console cable, the RS-232/V.24 signal can "float" and mimic the BREAK signal. This is why we advise that BREAK be disabled on console ports by default.

Most Access Server devices from any vendor typically send a small "glitch" of energy at one or more of these events:

  • The access server is powered on.

  • The access server is powered off.

  • A cable is attached/detached to the console port of the target device.

  • The serial controller hardware of the access server is reset.

This "glitch" looks like a "BREAK" signal. By default, Sun Microsystems computers will halt execution of the operating system and drop in to ROM monitor upon receipt of BREAK. This behavior is intended to facilitate debugging system hangs and serious performance issues. Other UNIX systems may have this behavior as well, but it is most often seen in Suns.

Problem Symptoms

Devices being console accessed from access servers are no longer accessible via any network ports.


Because the spurious BREAK signal is an artifact of physical layer issues, a solution is required that prevents the BREAK signal from getting from to the Sun, or that causes the Sun to not interpret the BREAK signal as a halt command. Though the following solutions are not officially supported by Cisco Systems, these have been shown to work at customer sites and are presented here as a courtesy:

  • Visit the web site below.

    The web site below describes the software patches needed to stop breaks. Solaris versions 2.6, 2.7, and 2.8 can be modified to stop breaks, but need the patches that are described at the web site below:

  • Install an in-line device that intercepts the BREAK signal.

    A model NUD4273 "non-aborting serial console adapter" from NUData will prevent the BREAK signal from ever reaching the console port. The devices cost about $89 USD each.

    (Nudata is at 32 Fairview Avenue, Little Silver, NJ 07739-1515. Tel: +1 732 842 5757, fax: +1 732 842 1161.)

  • The "Soldering Iron" solution

    Note: The first solution presented above, Visit the web site for software patches, is the preferred solution. The solution described below may not work for all devices.

    If you tie a 4.7K resistor (1/4 Watt) between pins 3 and 25 of the ttya port, you electrically prevent a BREAK signal either from the key or from disconnecting or powering down the terminal. This prevents intentional halts except by removing the resistor, but does allow recabling.

Verification of Code Version

All Cisco Access Servers with asynchronous ports that can be used to console access to other network devices.

For More Information

If you require further assistance, or if you have any further questions regarding this field notice, please contact the Cisco Systems Technical Assistance Center (TAC) by one of the following methods:

Receive Email Notification For New Field Notices

Product Alert Tool - Set up a profile to receive email updates about reliability, safety, network security, and end-of-sale issues for the Cisco products you specify.