Multiple processes using AdsDevice on Linux cannot connect to PLC simultaneously (connection resets / error 1861)

[pere-betarobots]

Description

We are running into an issue where two independent processes on the same Linux PC both need to connect to a Beckhoff PLC via ADS.

• When both processes try to use AdsDevice at the same time, only one of them succeeds.
• The other process immediately fails with Ads operation failed with error code 1861 or later with Broken pipe errors.
• If only one process is running, it works perfectly.

This makes it hard to design modular ROS nodes where each node is its own process but both need PLC access.

Environment

• OS: Ubuntu 20.04 (ROS Noetic environment)
• ADS library: 0.0.28-1
• PLC: Beckhoff TwinCAT 3 runtime (PLC running on 192.168.125.10)

What we tried

All this tests done with the PC IP set to 192.168.125.100

Working scenario (one process)

• Start one ROS node/process using AdsDevice with Local AMS Net ID 192.168.125.100.1.1, local ADS port 30000.
• Works fine: we can read/write variables without issues.
• Also works with .1.2 as long as it’s the only process.

Also working: multiple AdsDevice in one process

• Inside one process, create two AdsDevice objects:
  • 192.168.125.100.1.1 → port 30000
  • 192.168.125.100.1.1 → port 30001
• Both coexist, read/write works.
• Suggests that the library/router is multiplexing properly inside a single process.

Failing scenario: two processes

• Start two independent processes:
  • Process A → 192.168.125.100.1.1, port 30000
  • Process B → 192.168.125.100.1.2, port 30001
• Only one process connects successfully.
• The other process logs:

Error: read frame failed with error: Connection reset by peer
Failed to connect: Ads operation failed with error code 1861.
Error: write frame failed with error: Broken pipe

• Tried same AMS Net ID in both processes (.1.1 for both), and also different AMS Net IDs (.1.1 vs .1.2).
• In both cases, one process always gets dropped.

Question

What is the recommended way to support multiple independent processes on Linux connecting to the same PLC over ADS?

• Should we expect this to work with ads-lib (and we are misconfiguring something)?
• Or is the design such that only one process per host is supported unless there’s a central ADS router service?

Error logs (for reference)

[Beckhoff] Failed to connect: Ads operation failed with error code 1861.
Error: read frame failed with error: Connection reset by peer
Error: write frame failed with error: Broken pipe
[Beckhoff] Failed to connect: Ads operation failed with error code -1.

Would really appreciate guidance here.

Our use case: ROS nodes running in separate processes, both needing ADS connectivity. Currently this only works if we collapse everything into one process.

Thanks,

Pere

[pbruenn]

You are not the first to hit this, see:
#72 (comment)
#49 (comment)
#201 (comment)

In "standalone" mode this AdsLib does not support complex ADS client scenarios, due to the missing ADS router. However, you are lucky. TwinCAT on Linux is happening[1] and you can approach your local Beckhoff support to get access to the early access program.

[1] https://www.beckhoff.com/en-en/products/product-news/linux-r/

[pere-betarobots]

The links you shared really helped clarify the situation and confirm our suspicions that the missing ADS router is the root cause here.

In the meantime, as a temporary workaround, we’re going to try using ads-proxy to provide a single ADS client endpoint that can multiplex multiple processes on our side.

Thanks for pointing us in the right direction and for maintaining this project, really appreciate the support!

[tboegi]

You can try the ads-forwarder:

https://github.com/birkenfeld/ads_forwarder_rs

[aatb-ch]

You are not the first to hit this, see: #72 (comment) #49 (comment) #201 (comment)

In "standalone" mode this AdsLib does not support complex ADS client scenarios, due to the missing ADS router. However, you are lucky. TwinCAT on Linux is happening[1] and you can approach your local Beckhoff support to get access to the early access program.

[1] https://www.beckhoff.com/en-en/products/product-news/linux-r/

Hi, do you know if this means the XAR would be able to run directly on a linux machine? I have the exact same use case as OP, I have a ROS2 network with multiple nodes that need to interact with a CX plc, same issue. From your response you suggest that the TwinCAT runtime could run directly on the same machine? not on a linux PLC, since this would lead to the same problem? Wouldn't the "one ip = 1 connection" issue still apply?

[pbruenn]

In "standalone" mode this AdsLib does not ...
Hi, do you know if this means the XAR would be able to run directly on a linux machine? I have the exact same use case as OP, I have a ROS2 network with multiple nodes that need to interact with a CX plc, same issue. From your response you suggest that the TwinCAT runtime could run directly on the same machine? not on a linux PLC, since this would lead to the same problem? Wouldn't the "one ip = 1 connection" issue still apply?

What do I mean by "standalone" mode? This open source library was written for Linux systems without local TwinCAT. That's what I call standalone mode. Meanwhile we got users who want to use this lib on Windows, TC/BSD or soon even Linux with TwinCAT router installed. That's what I would call TwinCAT mode.

In TwinCAT mode you use the TwinCAT router directly there is no TCP/IP involved as long as you just communicate with local AdsDevices (like the PLC). And even if you would talk to a second remote PLC the TwinCAT router would do the TCP/IP multiplexing for you. Like it does when you have multiple TwinCAT XAE Instances on your Windows Laptop communicating with a TwinCAT PLC somewhere.

The "one ip = 1 connection" issue is only a problem of the standalone mode, when the TCP/IP connection is managed by the opensource AdsLib itself. We just didn't implement a router here (and we won't since now you could just install TwinCAT. The last remaining benefit of this open source library over the TwinCAT TcAdsDll.dll is you have a little more modern API (and you could write simple ADS Clients without TwinCAT).

[aatb-ch]

I see, thanks for the response!

[aatb-ch]

Hello, I just wanted to ask a last clarification regarding the TwinCAT router on linux. Installing TC including router on Linux will be possible exclusively on your new "Beckhoff RT Linux" debian-based distro, or would it be possible to have it on another distro like Ubuntu? I guess it might cause issues regarding realtime constraints, but just wondering if I will have to switch, since ROS2 provides binaries for Ubuntu it makes it quite convenient to use.. Ideally my ROS2 machine would stay on Ubuntu, so I wouldn't be able to use the native linux TC router?

[pbruenn]

Yes, you should be able to install the TwinCAT router easily on Debian based Distros. Even if you would run into issues like c++ runtime dependencies you always have the opportunity to run either TwinCAT or your other applications in a podman or Docker container. Of course real time performance will depend on you kernel config and hardware(CPU) partitioning.

You can find the Kernel source and config of Beckhoff RT Linux here: https://github.com/Beckhoff/linux

[aatb-ch]

ah, that's great, thanks a lot! then I'm looking forward to release, will ask my local beckhoff rep to see if I can have access to it for testing on my dev machine.