Gotchas

Timer/counter preset write returns E1

Field	Detail
Symptom	A timer or counter preset write returns E1.
Root cause	Host Link preset writes through WS/WSS are supported on KV-8000/7000-series, not on KV-3000, KV-5000, or KV-NANO.
Fix	Do not write timer/counter presets on unsupported models; use read_timer, read_counter, or read_timer_counter for safe reads.

import asyncio
from hostlink import HostLinkConnectionOptions, open_and_connect, read_timer


async def main() -> None:
    options = HostLinkConnectionOptions(host="192.168.250.100", port=8501)
    async with await open_and_connect(options) as client:
        timer = await read_timer(client, "T0")
        print(timer.preset)


if __name__ == "__main__":
    asyncio.run(main())

AT device fails on KV-X500

Field	Detail
Symptom	Reading AT0 fails or the range catalog reports no AT entry for KV-X500.
Root cause	AT is not available in keyence:kv-x500 or keyence:kv-x500-xym.
Fix	Check the selected profile before using AT, and avoid AT on KV-X500 projects.

from hostlink import device_range_catalog_for_plc_profile


def main() -> None:
    catalog = device_range_catalog_for_plc_profile("keyence:kv-x500")
    entry = catalog.entry("AT")
    print(f"AT supported: {entry.supported if entry else False}")


if __name__ == "__main__":
    main()

X or Y address rejected

Field	Detail
Symptom	X or Y raises an address parse or PLC device-number error.
Root cause	X and Y use decimal-bank plus hex-bit notation. X10F means bank 10, bit F.
Fix	Use X10F or Y10F, and select an -xym profile when you want XYM aliases.

from hostlink import normalize_address


def main() -> None:
    print(normalize_address("X10F"))


if __name__ == "__main__":
    main()

R, MR, LR, or CR address rejected

Field	Detail
Symptom	R, MR, LR, or CR raises an address parse or PLC device-number error.
Root cause	These families use KEYENCE two-digit bit notation.
Fix	Use forms such as R200 or MR100; do not treat the full suffix as one hexadecimal number.

from hostlink import normalize_address


def main() -> None:
    print(normalize_address("MR100"))


if __name__ == "__main__":
    main()

Wrong port causes an immediate timeout

Field	Detail
Symptom	The connection times out immediately even though the PLC responds on the network.
Root cause	KV Host Link uses port 8501, not the SLMP/Computerlink port 1025.
Fix	Set port=8501 in HostLinkConnectionOptions.

from hostlink import HostLinkConnectionOptions


def main() -> None:
    options = HostLinkConnectionOptions(host="192.168.250.100", port=8501)
    print(options)


if __name__ == "__main__":
    main()

Abnormal device number returns E0

Field	Detail
Symptom	A read or write returns Host Link error E0.
Root cause	The selected device address does not exist on the target PLC or is outside the range for the selected profile.
Fix	Check Supported registers and PLC profiles, then choose an address that exists on that PLC model.

from hostlink import device_range_catalog_for_plc_profile


def main() -> None:
    catalog = device_range_catalog_for_plc_profile("keyence:kv-7000")
    dm = catalog.entry("DM")
    print(dm)


if __name__ == "__main__":
    main()

Write disabled returns E4

Field	Detail
Symptom	A write returns Host Link error E4.
Root cause	The CPU is write-protected, locked, or blocked by the target project settings.
Fix	Check the CPU protection and KV Studio settings before retrying the write.

32-bit and float values look wrong

Field	Detail
Symptom	:D, :L, or :F reads return values that do not match the expected PLC-side value.
Root cause	The target words do not contain the data type you selected, or the ladder uses a non-standard layout.
Fix	Use :D for unsigned 32-bit, :L for signed 32-bit, and :F only when the PLC stores IEEE 754 float32 data.

Non-canonical profile string rejected

Field	Detail
Symptom	A short name, display label, or old alias raises HostLinkProtocolError.
Root cause	The range catalog accepts only exact canonical profile strings. It does not infer the profile from a PLC model response.
Fix	Store the canonical string from PLC profiles in your project settings or UI value.

from hostlink import available_plc_profiles


def main() -> None:
    for profile in available_plc_profiles():
        print(profile)


if __name__ == "__main__":
    main()