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NetDecoder - DH+ Analyzer:
Sniff, Analyze, Debug and Troubleshoot
DH Plus Networks
 The NetDecoder
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DH+ Analyzer is a comprehensive data capture, debugging, and analysis tool
for troubleshooting DH+ networks. Data Highway Plus (DH+) is an Allen
Bradley, Rockwell Automation, token passing data communications network.
This protocol consists mainly of Tokens, Solicits, ACKs, NAKs, Timeouts and
Datamessages. The device IDs are octal numbers running from a value of 000
to 077 octal. Tokens are passed in an orderly fashion from lowest ID to
highest ID with the potential for a single Solicit per complete Token
Rotation in order to pick up any additional devices on the network. Once a
token is received, the receiving device can place Data Messages on the
network (usually up to 4 at a time), wait for Acknowledgements or Timeouts,
send a Solicit to the next device on the solicit list or pass the Token to
the next Token Successor.
 The NetDecoder
analyzer currently uses the DHM-3500 device,
with custom firmware, to sniffer DH+ traffic. To run the software, users can
select this option from NetDecoder's start-up wizard menu.
The
NetDecoder DH+
Analyzer includes a DH+ statistical analysis module that provides critical
network statistics in a graphical, easy-to-comprehend manner that makes
debugging network problems easy. The statistical module is helpful when
starting up, surveying, benchmarking and troubleshooting DH+ networks. The
DH+ statistical module is also designed to be a continuous monitoring
feature that makes it easy to spot network problems in their nascent stage.
Early detecting and intervention will prevent small problems from escalating
into huge network downtimes.
 The DH+ statistics overview screen provides a
quick and easy to understand analysis of your overall network performance.
The additional screens are used in order to drill down deeper into the
individual device and conversation statistics. These screens can be captured
for use in reports by pressing the ALT and PRINT SCREEN keys and pasting
them into your reports. The information provided in the statistical analysis
module can be used to survey and benchmark existing networks, assist in the
commissioning of new networks or network expansions, and diagnose
intermittent device communications. All of this information can also be
exported to a csv file for further analysis or custom report writing from
the File Menu. The statistical analysis module can be run or rerun on any
previously captured DH+ network files. Keep in mind that if a capture file
is set to wrap, the original statistics are calculated over the entire
capture session as opposed to merely representing what has been captured to
disk. If you recreate the statistics from the File Menu, your original
session statistics will be overwritten to reflect only the data captured to
the disk file. Statistics files are named using the same name as your
capture file and a file extension of sts. It is possible to save both
session and capture file statistics by saving the capture file to another
name and then recreating the statistics from the File Menu.
 Individual
device statistics can be obtained by clicking on the device number box
in the grid, for the device of interest. This display will detail all
traffic to or from the device in question. Notice that the display
breaks the various conversations down into their individual components;
Commands, Replies, Acknowledgements, and Negative Acknowledgements. The
top-most table displays all of the communications statistics for traffic
that has originated from the device of interest. Conversely, the bottom
most table displays all of the communications statistics for traffic
that has taken place with this device but was originated by other
devices. Notice that response times are displayed here as well. These
can be used to determine worst case device performance. Note that in
general, the number of Commands, ACKs and Replies should be equal on a
well-behaved network. Due to the timing of the capture being started and
stopped, single digit number differences may occur but any differences
of 10 or more indicate a loss of communications between the devices in
question. It is also likely that one of the two devices in question will
have either a yellow, red, or blue box on the grid or a red triangle in
the upper right corner of their box, indicating a problem with that
device.
 Clicking
on the Log button or selecting View, Network Event Log from the DH+
Statistics Overview Screen will show a dialog with a breakdown of
various network events that are automatically logged. This information
provides additional details about the health of your network. This log
stores the most recent 2000 events and can be easily overrun by devices
being powered down if the logic is not written to consider this fact.
Large percentages of network bandwidth can be absorbed by messages and
retries directed at devices that are no longer on the network. The
non-responder and long responder markings on the device grid, and the
network event log will help to identify this kind of inefficiency. You
will notice that in the example below, Device 021 appeared on the
network and then took more than 5 seconds, but less than 60 to respond
to commands sent to it. An indication that this has occurred is readily
available on the Active Devices Grid, by way of the blue triangle in the
device ID box for device 021. This could be an indication that Device 21
is overloaded or is starting to fail. The Network Event Log provides
supplemental information regarding how often a particular device is not
responding or is a long responder.
Download the
DH+ Statistical Analysis Module Overview or download a
NetDecoder demo. You can also
order NetDecoder online. |