Tpu2000/Tpu2000R Modbus Ascii Communication Timing Analysis - ABB TPU2000 Technical Manual

Another method to troubleshoot the TPU2000/TPU2000R is to use the 03 (Read Holding Register) command and
access the communication status registers. The communication status registers reside at 4XXX through 4XXX.
Section 5 of this document list the method to access and use these registers.
Finally, it is always advantageous to use a datascope or a communication analyzer when troubleshooting a
Modbus Network. Such devices allow the implementers to view the data strings between the host and IED.
Modicon's parent company Schneider Electric has designed many utilities and products to assist the network
professional with troubleshooting.
guessing as to what is sent between a host and IED. Such tools available are at a modest cost, such as Modlink,
or at no cost MTS. Many of these tools are available on the website

TPU2000/TPU2000R Modbus ASCII Communication Timing Analysis

Perhaps the most common error in implementing a Modbus ASCII network is timing setup for communication.
Modbus ASCII protocol operates according to the following timing rules:
•
If the TPU2000/TPU2000R receives a command without a communication error (LRC,
PARITY, FRAMING, OVERRUN ... errors), a normal response occurs.
•
If the TPU2000/TPU2000R does not receive a command without a communication error
(LRC, PARITY, FRAMING, OVERRUN, Errors), no response is returned. The host (master)
device will sense a timeout according to its timeout parameter. The host could then send a
new command or retry sending the original command.
• Modbus ASCII allows for internals up to 1 second between characters are acceptable gaps.
The TPU2000/TPU2000R will not timeout. Character send gaps in excess of 1 second will
result in TPU2000/TPU2000R Modbus port timeouts.
TPU2000/TPU2000R Modbus network implementers will usually notice communication errors in the form of
excessive communication retries, errors, or non-responses. Understanding communication timing is a subject
rarely covered in protocol manuals, but an important topic in network implementation.
Network timing is predicated upon the following factors:
Host Latency (How long does it take a host device to generate a command, receive the response and
interpret the data).
Intermediate Device Latency (If a Modem, data concentrator or other device is between the end device
required for data retrieval, how long does it take for each device to receive the command and process it
downline to the next device).
TPU2000/TPU2000R Device Latency (How long does it take for a TPU2000/TPU2000R to receive a
command, and return a response to the network).
Baud Rate (How fast is each data bit propagated on the medium. One cannot get around the laws of physics)
Protocol Efficiency [Network Bandwidth Utilization] (Does the protocol utilized allow for the issuance of
another command before a response is received from an outstanding communication request).
The common question to a network system engineer is usually " How fast can I get my relay alarm data to appear
on the screen?". An analysis of the amount of data and the above 5 areas is required.
Host latency varies widely by manufacturer or the PC or host computer. Software speed and port access varies
widely. Most manufacturers of these hardware and software platforms have general benchmarks to supply to the
users for processing time once the device acquires the data from the communication port.
Intermediate Device Latency also varies from the type of device used. Some modems have a device turnaround
of 5 mS per transactions whereas, a radio modem may require hundreds of mS to obtain an open frequency from
which to transmit.
This section shall illustrate and explain a simple network transaction based upon a simple point to point
communication from a single TPU2000/TPU2000R to a host device as illustrated in Figure 5-75 of this document.
This example shall exclude SCADA Master host latency .
TPU2000/2000R Modbus/Modbus Plus Automation Guide
Such tools are inexpensive when compared to the person-hours spent
www.modicon.com .
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