Beckhoff TwinCAT ADS 1861 timeout when HMI reads PLC variables
What causes Beckhoff TwinCAT ADS error 1861 timeout when an HMI reads PLC variables over the network after a few minutes?
Top answer · Community question, shared from a troubleshooting session and reviewed by InduQT · Oct 27, 2026
An HMI is experiencing intermittent ADS 1861 (timeout) errors when reading PLC variables over the network. This indicates that the PLC or network is not responding within the configured timeout period, often due to performance issues or network congestion.
Likely causes and checks
1Target PLC or network too slow to answer within the timeout
- Points to it
- The error occurs 'after a few minutes' and is an ADS 1861 timeout, meaning the request was sent but no answer arrived before the client's timeout. This is a common symptom of intermittent issues under load or network congestion.
- Check
- Compare the HMI's request size and rate with the configured ADS timeout. Trace the response time and the task exceed counter on the PLC target around the time of the error to identify if the PLC is overloaded or if network latency is the issue.
2Firewall blocking ADS on the target PLC
- Points to it
- An ADS 1861 timeout can occur if a firewall on the target PLC is blocking ADS communication. The error occurring 'after a few minutes' rather than immediately makes this less likely than a performance issue, unless firewall rules are dynamic.
- Check
- Check the firewall rules on the target PLC to ensure incoming TCP port 48898 (and UDP 48899 for search, TCP 8016 for Secure ADS if applicable) are allowed.
3Incorrectly configured ADS timeout in the HMI client
- Points to it
- While an incorrect timeout setting can cause ADS 1861, the error occurring 'after a few minutes' suggests the default timeout might be too short for specific network or PLC load conditions, rather than being fundamentally misconfigured.
- Check
- Verify the configured ADS timeout in the HMI client (e.g., AdsSyncSetTimeout or TMOUT for ADSREAD/ADSWRITE blocks) and consider increasing it if the network path is long or the PLC is heavily loaded.
Next check: If the issue persists after checking the above, consider capturing network traffic to analyze packet loss or delays.
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