I do not receive a connection fail. What do the black boxes represent?

 

You're looking at the Traffic view in ModScan. The black boxes are used to differentiate between requests and responses. The bytes in the black box are the response packets.

It appears that your Modbus/TCP Server device is only responding to some of ModScan's requests. And the responses that are being sent back are exception responses, specifically exception code 0x0B - GATEWAY TARGET DEVICE FAILED TO RESPOND. From the spec, this exception means the following:
Specialized use in conjunction with gateways, indicates that no response was obtained from the target device. Usually means that the device is not present on the network.

What is the device that you're communicating to? Is it a type of Modbus gateway, router, or similar?

 

You're looking at the Traffic view in ModScan. The black boxes are used to differentiate between requests and responses. The bytes in the black box are the response packets.

It appears that your Modbus/TCP Server device is only responding to some of ModScan's requests. And the responses that are being sent back are exception responses, specifically exception code 0x0B - GATEWAY TARGET DEVICE FAILED TO RESPOND. From the spec, this exception means the following:
Specialized use in conjunction with gateways, indicates that no response was obtained from the target device. Usually means that the device is not present on the network.

What is the device that you're communicating to? Is it a type of Modbus gateway, router, or similar?

I was trying to scan it remotely through a serial to ethernet converter. It is a lantronix xsdrin-03. When I connect directly to the serial port of the analyzer, I get Modbus exception response from slave device.

 

OK. I'd suggest simplifying and go back to directly connecting to the serial port of the analyzer and get that working first before moving on to connecting remotely through the serial to Ethernet converter. Do you know what exception response you are getting? In the traffic view of ModScan, it will be the byte just after [83], assuming you are viewing in binary or hexadecimal.

 

OK. I'd suggest simplifying and go back to directly connecting to the serial port of the analyzer and get that working first before moving on to connecting remotely through the serial to Ethernet converter. Do you know what exception response you are getting? In the traffic view of ModScan, it will be the byte just after [83], assuming you are viewing in binary or hexadecimal.

I will head out to the field tomorrow and let you know. I can not remember right now.