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Console Server BREAK-Off
version 1.3 (updated:May 06, 2009, 10:17 pm )

Questions about testing? email break-off at

This document is the original page from when we planned to begin testing, and was updated as testing progressed. Other pages detail our testing results (which servers passed our tests, and which didn't), our test methodologies, and credits and acknowledgments.

Are you curious why I do this? The information from testing has benefited my employers and their customers, but I post the information freely to the web, in the hopes that it will help others as well. Remote Serial Console Access still seems to be one of the best kept secrets of Sysadmin and Netadmin...I'm out to change that. I don't charge for testing products, and the vendors who participate get some feedback on their product, user interface, packaging, documentation, sometimes their support services, and other aspects of the customer experience. In return, I get more experience with the newer products, and other folks get the benefit of comparing my results without needing to perform the testing themselves. It's a win-win-win situation. 

Equipment desired for the testing

  • A digital oscilloscope, capable of printing/exporting waveforms?

I did get a terrific serial line protocol analyzer from Benedict Communication, their model DLM-200. (The DLM-200 page is no longer there, but you can see the bigger brother, the DLM-250, which adds more buffer RAM, and on-board LEDs for monitoring the signal leads.)

My unit was invaluable for retesting many devices, when I lost my 8-year-old database of serial info. This analyzer is in my Serial Doctor Bag that's with me most of the time. I'm very pleased with it's performance, and with Benedict Computing.

Testing Results Summary
(Hardware I've tested that doesn't send unexpected BREAK)

Cisco NM-16A, NM-32A
Cisco HWIC-8A, -16A
Computone RAS2000
Cyclades TS and ACS
DNPG DECserver
Digi CM
iTouch InReach 8000
Opengear CM
Perle CS9000

Detailed info is available on the BREAK-off Test Results page.

Why do we care?

We consider remote access to serial ports to be a very valuable tool in administering many large enterprise networks, and many of our peers do as well. To that end, we normally deploy a logging console server application at sites that we service, and we have also deployed a number of terminal servers over the years to extend our administration reach throughout those networks.

Unfortunately, many terminal servers have this unexpected serial BREAK problem. More problematic is the fact that many of the large sites we support use Sun workstations and computers;

  • Sun servers usually respond to serial BREAK by going to the OK prompt.
    • Most operations are suspended, rather than being killed.
  • Older Terminal Servers can send BREAK to all attached devices at inappropriate times.
    • Some terminal servers can support 192 you could halt 192 Suns at once...
  • Once halted, you have to get on each port and type GO to get things running again.
    • Each server can support thousands of simultaneous users, who may lose work...
    • You need to wait for the terminal servers to accept remote connections again.
    • It takes time to remotely access each port, one at a time.
    • It's hard to automate this process.
  • There are various ways you can inhibit a SUN from detecting, or responding to, the BREAK signal.
    • As a Sysadmin, sometimes you need to use BREAK, to get the machine to respond to you, so making it ignore the signal would be Bad.
    • Some fixes are not available on older versions of OS.
    • Some fixes are not available unless you have a current support agreement.

Simply put, we don't want to give up the benefits that remote access to serial consoles brings us, so we want to know which terminal servers won't cause these problems. As a result of that need, we're putting a bunch of different equipment through the mill, and telling the Web what we find out.

Shameless plugs: Last year, SysAdmin Magazine produced a special Console Server supplement with their September issue. (There was another Console Server supplement with the May 2006 issue, but I didn't contribute to that issue.) Look back to the September/October 2000 issue of ;login: for an article about deploying remote access to console ports. Also, the current keeper of the Conserver application and I taught a half-day tutorial (M12) at the LISA 2000 conference in early December, on deploying the Conserver application, and using Terminal Servers for remote access to serial consoles. I taught again at LISA in 2002.

What we are doing to test these theories:

We started this project because getting information on whether or not a device has the serial BREAK problem was more like tracking down an urban legend than science. Even with high confidence in a device, you can usually find someone who will suggest that some unidentified third party has had some trouble with that same device in the past...

Our lab testbed consists of a small IP subnet, a target host (Sun SPARC 5 and a Sun Netra T-1 105), a Conserver (HP Laptop running Linux, or an iMac runnning OS X) which can also be a TFTP and applications host, a handful of terminal server units under test, and another system or two as a Conserver client and running a terminal application to use the serial port for additional diagnostics, as well as pinging the target host.

In our lab, we have divided the duties a bit, so that the Server performs the Conserver and TFTP duties, while another computer performs most of the terminal emulation duties.

  1. Server
  2. Target
  3. Laptop
  4. DUT #1
  5. DUT #2
  6. DUT #3

All of the devices are connected to a common ethernet segment. The Laptop has a serial port (shown as the green lines in the illustration above), which we use to communicate with many of the Devices Under Test (DUT) for their initial configurations. We then connect each DUT in turn, via serial (shown in the illustrations as an orange line) to the target Sun machine, and then perform the various tests, and see if the host is brought to the OK prompt.

In one case, we shared our test methods with a team at another site (Jeff Komori and Jim Dumont at Synopsys), and asked them to test some hardware that they could not loan us.

The result is a very high confidence in the Cisco 3600 series devices (3620, 3640, 3660) and the Cisco 2600 series devices (2621 tested) with the NM-32A modules. They have not sent a detectable BREAK signal under our test conditions.

I do realize that good science relies on larger sample pools. We are continuing to solicit loaner equipment that we can put through our testbed. As we run out of easy access to equipment, we will begin to accept more offers from other teams at universities and other large sites to perform this testing on their own gear using our test methods. We'll review their test results, and add those results to our database as 'external data' submissions, in order to identify which testing we have performed personally. The initial test run is intended to help prove out how many devices have software-related causes versus hardware-related causes, and which devices do not exhibit the problem.

During BREAK testing of products, I'm willing to share product specifics with my vendor contact prior to publication of my test report. (That is, I'll tell Cisco what we found about the Cisco gear, tell Nortel about the Nortel/Bay/Annex gear, etc.) However, I'm interested whether anyone thinks that would be a bad idea (and why), so I can plan accordingly, and set appropriate expectations with the vendor contacts. So far, I think this method has worked very well.

I have prepared a F.A.Q. page for this testing project, if you want to know more about the processes used to get products in to test.

More about BREAK

Conserver allows remote client users to invoke the terminal server to issue a BREAK on individual serial ports. This is one of the many features that makes Conserver useful, and popular.

Sun offers an option to tell the CPU to ignore BREAK over the serial console. However, there are times when you NEED to send a BREAK to the CPU. If you have used this option, then you need to keep a monitor and keyboard attached to the unit in order to force that BREAK when you need it. (Conserver is intended to help free up computer room space by allowing System Administrators to remove those extra keyboards and displays.)

Some terminal server vendors reportedly include a mode, or line settings, which will keep the serial ports from sending BREAK. While it may keep the port from sending the signal at inappropriate times, if that mode/setting also keeps Conserver from sending the BREAK signal when we need to, this would not be a useful option.Cisco reportedly has an ECO in place, which modified the main board inside the 2511 product to address the BREAK issue. I'm still looking for info about how you can tell without testing if your units should or shouldn't send BREAK.

Cisco reportedly had an ECO in place, which modified the main board inside the 2511-RJ product to address the BREAK issue. (More info on our testing result page.)

Cisco also refers to a NuData device (NUD-4723 non-aborting serial cable) that you can attach in-line between serial consoles and the terminal server. The devices are $100 (US), and used to be available through NuData at 1-800-844-5757. (The *-warehouse vendors no longer carry this part.)

ASP Technologies manufactures a commercial console server application called Vantage, but they also manufactures some interesting hardware dongles that prevent certain devices (Digi STS-1600, Xyplex terminal servers) from sending serial BREAK! This is different than the NuData RS-232 in-line BREAK filter which is referred to in the Cisco tech notes. For the Digi units (and others?) it is a dongle that connects between the power supply and the SCSI-attached pod. For the Xyplex, you need to use a soldering iron after opening the case, but this could be cheaper and easier than spending $100 US per port for the NuData devices.

Test Methodologies

The BREAK signal is an inversion of the data lead for a specific period of time, exceeding the period of one character, including the start/stop bits, and any parity. As such, the duration of the BREAK signal is a function of the port speed of the UART sending the data. However, many UARTS will send a BREAK signal that is 4 times as long (some are 16-times as long) as a single character. Some DEC manuals state that the period is actually ~250 milliseconds, and many VT-terminals will send a long BREAK signal.

Simple visual observation may be possible with a passive RS-232 signal tracer for most devices. This can tell us whether the BREAK occurs immediately after Power Off or Power On, or if it occurs later in the boot cycle, during a warm-boot of the device, or a reset of the serial interface, and by invocation from Conserver. (Sun devices are sensitive to short BREAK signals. If your port speed is set to higher than 9600 bps, it can be difficult to see a short BREAK signal. For our testing, we set the port speeds to 9600 whenever possible.)

Other testing includes measuring the actual power output of the RS-232 drivers. While the specification calls for up to +/-25VDC for the low/high logic signals, the "no-man's land" range (where "data is invalid") is down at +/-3V. It is possible for devices to output less power, but then they won't drive a signal as far...and we might care about that when we consider large data center installations. Since some devices use newer I.C.s with charge-pump technology, its possible to see levels closer to +/-6V or +/-9V, and still work OK except in some edge cases with long cable runs. Most devices so far have voltages between +/- 13V to +/- 8V.-

If the power rails are to blame, the signal could last longer than a normal BREAK. If this is the case, it is possible that the Sun CPU is "too sensitive" to BREAK. That is, if the signal doesn't return to normal after the prescribed time period (as I believe a power rail-induced failure would cause), the signal should probably be ignored, rather than acted upon.

If this is a power condition-related failure, we would likely need a digital oscilloscope, or similar device that could trigger on a signal event, capture the waveforms of the TXD and RXD leads (voltage over time), as well as perhaps the logic power rail voltage, and then export that display to a Polaroid photo, a printer, or an image file on a floppy, so that we could include the image in our slides and write-ups.

NOTICE: Most of the pages, articles, and tutorials on this website are copyrighted works. You may make 'deep links' to various pages. (If you let me know which page(s) you are linking to, I'll let you know if I move the page(s) during updates.) Please send me email if you wish to republish any material, or use it on your own website.

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Copyright 1996-2009, David K. Z. Harris, N6UOW   View David K. Z. Harris's profile on
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