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Old September 14th, 2021, 02:07 PM   #1
RY_Guthrie
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modbus poll

i am using modbus pol to trouble shoot a modbus comms issue. I have device communicating modbus over rs485 going through a lantronix express DR which convert is to modbus over tcp/ip. I can read the data fine on the rs-485 side and to the best of my knowledge the Lantronixs is setup correctly??? I dont have the software to check it. But when i connect modbus pol to the ethernet side I get Insufficient bytes received? I have tried multiple setting in modbus pol with no luck. I have the correct IP as I can ping the lantronix. I am leaning toward a lantornix config issue, but I have little experience with these devices so I was wondering what that error message even means. MOdbus pol gives no explanation in the users manual. Thank you for any advice!! Also is there somewhere online where you can download the lantornix software as I dont have access to the DVD? tired there webs site with no luck.
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Old September 14th, 2021, 02:43 PM   #2
diat150
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you should be able to telnet into the lantronix using the ip and port 9999


I dont think the express dr supports modbus tcp, you would need to use modbus rtu over tcp/ip in modbus poll. But first things first, you need to check all of the settings.
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Old September 14th, 2021, 03:23 PM   #3
Lare
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Quote:
Originally Posted by diat150 View Post
you should be able to telnet into the lantronix using the ip and port 9999


I dont think the express dr supports modbus tcp, you would need to use modbus rtu over tcp/ip in modbus poll. But first things first, you need to check all of the settings.

Thinking same, it is modbus rtu over TCP not Modbus TCP.


There is virtual com port software for PC. (You setup your PC to connect virtual com port it then redirects rtu querys to lantronics ethernet port if your client don't support modbus rtu over TCP protocol directly.)



Download link and setup software links are here:

https://www.lantronix.com/products/x.../#tab-overview
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Old September 14th, 2021, 03:38 PM   #4
Ken Roach
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I think the important distinction is "Modbus / TCP" versus "Modbus RTU encapsulated over TCP/IP".

Ethernet/Serial gateways that are "Automation protocol enabled" generally run real Modbus/TCP protocol on the Ethernet side. They receive a Modbus/TCP command, break it up into its component parts, and then send it out the serial side as a Modbus RTU command.

But there are also cheaper and simpler Ethernet/Serial gateways that don't know anything about industrial automation protocols. They just implement a "virtual serial port", and encapsulate the whole serial string or protocol frame instead of translating or interpreting it.

Lantronix "DR" are their Din Rail mounted gateways, of course. But they offer two nearly identical models: "XPress DR" and "XPress DR-IAP".

If you've got an XPress DR-IAP, then setting up your Modbus Poll for Modbus/TCP mode is correct.

But if you've got an XPress DR, then you need to set up Modbus Poll as though it was running Modbus RTU protocol over a virtual serial port, and get that virtual serial port configured using LanTronix tools.
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Old September 14th, 2021, 04:13 PM   #5
RY_Guthrie
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software

Unfortunately i dont have the install disc for the lantronix software so I am using the web interface. It is a xpress DR going by the id panel on the side. I just got the web browser working. I have not figured out what parameters are important and what ones are not. Its a new piece of equipment for me so I am having to read the manual. Thank you to every one who responded, this site has helped me more times than I can count!!
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Old September 15th, 2021, 03:37 AM   #6
lfe
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Maybe you have a problem with the RS-485 flow control, maybe the Lantronix module switches to receive mode too late and bytes are lost.

Do you know if the missing bytes are from the beginning or the end of the slave reply ?
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Old September 15th, 2021, 08:40 PM   #7
Psnikta
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The "Insufficient bytes received" message is probably because it is missing the MBAP header (7 bytes long)...


Does modbus pol display the messages in HEX?
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Old September 16th, 2021, 04:55 AM   #8
tragically1969
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I have nothing else to add apart form what Ken Roach said! You won't get anything on Modbus TCP from the DR model.
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