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!