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Your homebrewed software capture does not show any TCP packets. Your software is not establishing a connection and is not communicating with the MC 750 at all. You need to rewrite your homebrewed software. The .NET library I sent a link for is one way you could do this, and is likely the...

It appears the Wireshark captures are only of Modbus Poll, is this correct? Are you able to capture packets of your software sending transmissions to the MC 750? As a general recommendation, I suggest you look into using an existing .NET Modbus library for your software, such as the following...

Do one of the following: 1. Remove R7 (120 ohm termination resistor) from the MAX485 board and any other termination resistors that are applied or enabled on the other RS-485 devices. 2. Apply idle biasing resistors, one between A and VCC, and another between B and GND. The resistors should be...

You should only enter the Modbus packet into your software. In your images, you've also included Modbus Poll's packet ID and TX indication. For example, instead of entering: 003007-TX:0A CF 00 00 00 06 21 04 00 9D 00 02 You need to enter: 0A CF 00 00 00 06 21 04 00 9D 00 02 Regarding port...

As I said before, I think that the termination without idle biasing is what's causing your communication issue between the Arduino/MAX485 and the inverter. Here are two ways to resolve your issue. In either case, you need to connect A, B, and GND from your MAX485 board to the inverter's Data +...

As for when termination is required, here is a general guideline: Baud Rate Maximum Length Without Termination 9600 and below Termination not required 19,200 2,800ft (850m) 38,400 1,400ft (425m) 57,600 940ft (285m) 76,800 700ft (210m) 115,200 470ft (140m) The...

Great! Your calculations look correct to me. You should be able to confirm the values using the meter's display. Your next step is to try this with your homebrewed software. You can enter one of the TX packet's shown in Modbus Poll (one that has the 21 unit ID, not the 01 unit ID) in your...

Your USB/RS-485 converter likely uses a different RS-485 transceiver and has its own open-circuit biasing resistors that raise the idle voltage enough for the inverter to communicate with it. It's possible this biasing is enough to make your Arduino be able to communicate with the inverter when...

I apologize, but I don't understand this statement. Are you asking me for more explanation or have you asked ABB technical support for more explanation?

I'm sorry I didn't make myself clear. I was referring to S1:1 in this post. From the name, "BIAS_ENA", I suspect when this switch is in the ON position, the inverter connects biasing resistors to the RS-485 + and - lines. This image in the manual is actually incorrect. The "Inverter n" should...

I just took a look at what I believe is the manual for the inverter you're using https://seasolargroup.com/wp-content/uploads/2018/07/EN_PRO-33_0-TL_PM_B_A5_2015-06-18.pdf It appears that the inverter may have a switch to enable biasing on the network (in addition to a termination resistor...

It is very possible that your communication issue is due to the termination resistors without using biasing resistors. Try adding biasing resistors and see if you still have issues. So that I understand the results of your test, please confirm the following. There were 3 steps to my procedure...

When you have the length set higher than 1, does ModScan show this at the top? ** MODBUS Exception Response from Slave Device ** Can you read all 4 of the registers (5430,5431,5432 and 5433) individually by keeping the length at 1 and changing the address field? If you cannot read all 4...

First off, let me just say that the MC 750 is one of the most difficult Modbus devices to work with due to its unusual data formats, so I applaud your patience and perseverance in working with this device as an introduction to Modbus. It appears that you are correctly reading the voltage...

Well, ideally you would remove the resistor, but since that requires solder work, perhaps it's easier to connect idle biasing resistors between 5V and Data A and between Ground and Data B as shown in the Example RS-485 Network Diagram here: https://arduinoinfo.mywikis.net/wiki/RS485-Modules The...

You're quite welcome. Glad to help. I also wanted to point out another thing that can cause devices to be unable to communicate, which is if you have termination resistors on the bus without applying appropriate idle biasing (note that some devices have built-in termination resistors that can...

Yes, not connecting an RS-485 reference wire (a.k.a. common/ground) can cause devices to be unable to communicate. If the grounds are very different, the RS-485 circuitry of the devices could even be damaged. An important thing to note is that RS-485 reference/common/ground is NOT the same as...

Yes, establishing communications between Modbus Tools on your PC and the MAX485/Arduino should be done with the inverter powered off. Both the Modbus Tools simulator and the inverter should be configured for the same Slave ID, so you cannot have both on the RS-485 network at the same time.

One additional note regarding wiring. RS-485 actually requires 3 signals, not just 2 (+, -, and ground). Please make sure that all of your devices (USB/RS-485 adapter, MAX485/Arduino, and inverter) share a common ground connection.

OK, let's try to rule out the hardware and your connections. Can you please try the following? First, connect your USB/RS-485 adapter to the inverter and establish communications using Modbus Poll. After you have confirmed successful communications between your PC and the inverter, close Modbus...