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How to integrate different modbus devices on a web system?

I have two PLC modbus devices. These two are different companies.
The first one is that A will return the temperature and humidity. The second is that B will return five values. In both devices, the connection is modbus TCP.
One problem is that the same test software can be connected but the other cannot be connected. One device can be connected using pyModbus, but the other device is not working.
I also tested some software and python libraries. List is as follows
Device A
Software
modbus Poll (OK)
ModScan32 (NO)
Python Lib
pyModbus (OK)
EasyModbus (NO)
Device B
Software
modbus Poll (NO)
ModScan32 (OK)
Python Lib
pyModbus (NO)
EasyModbus (OK)
I don't know the difference between the two devices.
I want to integrate these two into the web system for monitoring, and new devices can be added in the future.
If there is a third device, I have to retest the connection to see which Python lib is available.
How can I implement this?
Learn more about the communication format of PLC devices?
Write a set of Python Libs for all devices?
Thank You.

Use one library for each, save the data somewhere centralized and view it with a different app. I don't see the problem. Also, have you tried re-trying the connection or reading after a few seconds? In my experience, modbus devices fail every so often to answer (or maybe is a library problem, I don't know) and a retry usually works.

For this, and it sounds like any good IOT framework would solve it, you should send the data from each device separately to one central point, like an IOT cloud solution, then you can build a webapp to view whatever data you have on the IOT framework, regardless of when or if it came in. Most frameworks will show you the old data as well if its not connected, so you can still display data, indicating it is stale or something to your user.

Workaround to open COM port for serial communication on client-side (preferably in Python)

Goal
I have made a website using the Flask framework and am fairly comfortable with HTML, CSS, JS, Python. My goal is to connect an arduino to the client's PC's usb port and use serial.write() to send a number to it.
Notes
I have a working example of interfacing with python if arduino was connected to the server.
import serial
ser = serial.Serial('COM4', 9600)
ser.write('5')
Now I want to run these exact 3 lines on the client side.
Is this even doable? I have researched a lot and it seems that this is not doable due to security reasons? (I'm hoping somebody proves me wrong here.) That is why I'm looking for a workaround. But before that I must mention, I don't need any of the data (numbers) to come from the server. Once the webpage is loaded all serial communication that I need is on the client side.
Client side python: I have looked into writing python on the client side and read about skulpt and PyPyjs but am not sure how I could run the mentioned 3 lines with them on client side(neither seems to support pyserial needed for import serial or at least I have not had any luck finding documentation)
I also looked into arduino documentation for interfacing with software but it seems that all the languages mentioned are server side. If you know of any possible direction languages that could help, I'd be happy to know and go learn them. I saw many forums mentioning Node.js but my understanding is that would also only do the job on the server side.
I'd appreciate any help on where else/other topics I should look into. Thanks in advance.

Is this even doable? I have researched a lot and it seems that this is not doable due to security reasons?
You are correct. There is no ability for browsers to access a COM port. It doesn't matter what language or framework you pick, a browser isn't going to give you that access.
You would need to make a standalone desktop application. You can use HTML and JavaScript to access serial ports, just not in a browser. Chrome Apps (which are actually going away) can do it, as well as an App that uses Electron.

Python, Rpi server status notifier

--First month in programming; be gentle with me--
I'm looking to build a short application using python to run on an RPi; the idea is to ping our company owned servers individually and eventually have them return as LED status lights. For now though; I would like it to broadcast a desktop notification to specific Macs on the same network.
I have 0 experience with python or programming general. Where should I start?

For learning Python there are too many very good web ressources easily found on the net, so I don't mention them here. For your nice little project for the Raspi you should get familiar with the python module called RPi.GPIO. With it you can easily turn on & off your LEDs depending on the ping response of your company servers.

How can I communicate between a Siemens S7-1200 and python?

I am running a process on a S7-1200 plc and I need it to send a start signal to my python script, after the script is done running it needs to send something back to the plc to initiate the next phase. Oh, and it has to be done in ladder.
Is there a quick and dirty way to send things over profibus or am I better off using just a RS232 thing?

I would go with SNAP7 and python-snap7 libraries. Example on how to install it on a Raspberry pi found in this blog:
http://simplyautomationized.blogspot.com/2014/12/raspberry-pi-getting-data-from-s7-1200.html
Update:
Video walkthrough on how to set it up: https://youtu.be/yJNEsI5KJxs

After failing with libnodave and OPC, I created a TCON,TSEND and TRECV communication thing. It transmits a byte over TCP and it works.

I would use libnodave on PC side to change value of some M tag or a DB tag in a PLC. You can easily make a ladder rung which will monitor this tag and execute further actions. Although libnodave doesn't support Python natively, you can find a python wrapper here.

Well, a quick google of 'python rs232' turned up some site talking about doing robotics controls through rs232 using pySerial. So if you know how to hook your widget up to rs232, you could go that route.

There is a commercial library called "S7connector" by Rothenbacher GmbH (obviously it's not the "s7connector" on sourceforge).
It is for the .NET framework, so could be used with IronPython.
It does work with S7-1200 PLCs. You just have to make sure a DB you want to read from / write to is not an optimized S7-1200 style DB, but a S7-300/400 compatible one, an option which you can set when creating a DB in TIA portal.
This lib also allows to read and write all I/O ports - the "shadow registers" (not sure what they're called officially) and directly as well, overriding the former.

There are several Modbus TCP Clients available for Python, I have used pyModbusTCP successfully. TIA Portal the IDE for the S7-1200 has Modbus Server function that you can load to your PLC that will allow you to read and write directly to the PLC.
https://support.industry.siemens.com/cs/ww/en/view/83130159

Ther best way to communicate with S7-1200 PLC cpu's is with OPC UA or Classic OPC (ommonly known as OPC DA. ) Libnodave is made for S7-300 and S7-400 not for S71200 (2.x firmware).
If you use a third party solution to communicate with S7-1200 (or S7-1500) you have to decrease the security level at the PLC by allowing the put and get mechanism. Put and get are pure evil to use. You open the memory of the CPU for every process. Don’t use them anymore. Siemens should actually block this.
This applies for all firmware release for S7-1200.
Siemens pushes people you use OPC UA as default communication from PLC. What makes sense, because OPC UA is the protocol for industry 4.0 and IIoT.
Edit: rewrite everything. Info was heavily outdated.
If you use a firmware 2 or 3 1200, consider replacement or upgrade. These versions are no longer supported and contains the worm issue.

Creating connection between two computers in python

The question: How do I create a python application that can connect and send packets over the internet to another computer running the same application? Is there any existing code/library I could use?
The background: I am pretty new to programming (HS senior). I've created a lot of simple things in python but I've recently decided to start on a bigger project. I'm considering creating a Magic: the Gathering booster draft simulator, but I'm not sure if it is feasible given my skill set so I'm asking around before I get started. The application would need to send data between computers about which cards are being picked/passed.
Thanks!

Twisted is a python event-driven networking engine licensed under MIT. Means that a single machine can communicate with one or more other machines, while doing other things between data being received and sent, all asynchronously, and running a in a single thread/process.
It supports many protocols out of the box, so you can just as well using an existing one. That's better because you get support for the protocol from 3rd party software (i.e. using HTTP for communication means middleware software that uses HTTP will be compatible: proxies etc.)
It also makes easy to create your own communication protocol, if that's what you want.
The documentation is filled with examples.

The standard library includes SocketServer (documented here), which might do what you want.
However I wonder if a better solution might be to use a message queue. Lots of good implementations already exist, including Python interfaces. I've used RabbitMQ before. The idea is that the computers both subscribe to the queue, and can either post or listen for events.

A great place to start looking is the Python Standard Library. In particular there are a few sections that will be relevant:
18. Interprocess Communication and Networking
19. Internet Data Handling
21. Internet Protocols and Support
Since you mentioned that you have no experience with this, I'd suggest starting with a HTTP based implementation. The HTTP protocol is fairly simple and easy to work with. In addition, there are nice frameworks to support this operation, such as webpy for the server and HTTPLib from the standard library for the client.
If you really want to dive into networking, then a socket based implementation might be educational. This will teach you the underlying concepts that are used in lots of networking code while resulting in an interface that is similar to a file stream.

Also, check out Pyro (Python remoting objects). Se this answer for more details.
Pyro basically allows you to publish Python object instances as services that can be called remotely. Pyro is probably the easiest way to implement Python-to-python process communication.

It's also worth looking at Kamaelia for this sort of thing - it's original usecase was network systems, and makes building such things relatively intuitive.
Some links: Overview of basic TCP system, Simple Chat server, Building a layered protocol, walk-through of how to evolve new components. Other extreme: P2P radio system: source, peer.
If it makes any difference, we've tested whether the system is accessible to novices through involvement in google summer of code 3 years in a row, actively taking on both experienced and inexperienced developers. All of them managed to build useful systems.
Essentially, if you've ever played with unix pipelines the ideas should be familiar.
Caveat: I wrote major chunks of Kamaelia :-)
If you want to learn how to do these things though, playing with a few different approaches makes sense, and you should definitely check out Twisted (the standard answer to this question), Pyro & the standard library tools. Each has a different approach, and learning them will definitely benefit you!
However, like nosklo, I would recommend against using the socket library directly and use a library instead - simply because it is much much harder to get sockets programming correct than people tend to realise.

Communication will take place with sockets, one way or another. Just a question of whether you use existing higher-level libraries, or roll your own.
If you're doing this as a learning experience, probably want to start as low-level as you can, to see the real nuts and bolts. Which means you probably want to start with a SocketServer, using a TCP connection (TCP is basically guaranteed delivery of data; UDP is not).
Google for some simple example code. Setting one up is very easy. But you will have to define all the details of your communications protocol: which end sends when and what, which end listens and when, what exactly the listener will expect, does it reply to confirm receipt, etc.