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Troubleshooting Guide

This guide provides solutions for common connectivity and protocol issues encountered when using the hostlink Python library.

Connection Issues

HostLinkConnectionError: Failed to connect

Possible Causes: - PLC IP/Port mismatch: Verify the PLC's IP address and Port (default 8501). - Network Firewall: Check if your OS or hardware firewall blocks TCP/UDP 8501. - PLC Config: Ensure "Host Link (Upper Link)" is enabled in the Ethernet unit settings in KV Studio. - TCP/UDP Mismatch: The library and PLC must use the same transport protocol.

HostLinkConnectionError: Timeout

Possible Causes: - Cable Issue: Check the Ethernet cable and hub. - Wrong IP: The target IP might not be reachable. Try ping <IP>. - Busy PLC: High-frequency communication might saturate old Ethernet units. Consider lowering the request frequency or using UDP.

Protocol Errors (HostLinkError)

E1: Abnormal command

Occurs when: The PLC received a command it doesn't support or understand. Solutions: - Bit Update Scope: The user-facing helper layer supports bit-in-word updates such as write_bit_in_word(client, "DM100", 3, True). Direct coil-force workflows are advanced operations and are not covered by the user guide. - Model Limitation: Some older CPUs do not support certain commands (e.g., URD expansion access). - Format Mismatch: Using .D or .L suffixes on devices that don't support 32-bit (rare, but possible).

E0: Abnormal device No.

Occurs when: The specified device address does not exist on this PLC. Solutions: - Out of Range: Accessing DM99999 on a CPU that only has 32,768 words. - Invalid Prefix: Using prefixes not supported by the model (e.g., EM or FM). - Timer/Counter: Accessing T0 when it hasn't been used or initialized in the ladder program.

E4: Write disabled

Occurs when: The CPU is locked or memory protection is on. Solutions: - CPU Lock: Check if the PLC has a memory protection switch or password set in KV Studio.

Unexpected Data

Value Mismatch (Endian Issues)

Symptoms: Reading 32-bit (.D/.L) values gives strange results. Analysis: Keyence stores 32-bit values with the Low-word first (e.g., DM100=Low, DM101=High). Solutions: The library handles this automatically when using .D or .L. Do not try to manually swap words unless you have a non-standard ladder configuration.

Float Handling

Note: The library supports .F for float32 helper reads and writes. Solutions: Use .F when the PLC side stores the value as IEEE 754 float32. If the device actually contains raw integer bits, read it as .D or .L instead.