Gotchas

Use this page only for library-specific caveats.

Shared SLMP setup, profile, point-limit, and end-code symptoms live in the shared SLMP Troubleshooting & Codes page. For profile limits and device availability, use the shared SLMP Profile Parameters page.

Current library-specific caveats

Area	Symptom	Guidance
IPv6 endpoint	connect() rejects an IPv6 literal or the supplied Arduino transport rejects a hostname.	The library contract is IPv4-only. Supplied Arduino TCP/UDP transports require a numeric IPv4 literal. A custom transport may support hostnames only when it resolves and connects to IPv4 without IPv6 fallback.
Diagnostics	Bytes are received, but the response is rejected as an invalid SLMP frame.	Verify the PLC Binary data-code setting and inspect lastRequestFrame() / lastResponseFrame() with formatHexBytes().
Buffer sizing	A request fails locally with BufferTooSmall.	Size TX for the complete 3E/4E request frame and RX for the larger of the maximum success response or PLC error-information response. Caller output arrays are checked independently. Random-label reads must supply their maximum encoded response-data bound. The rejection occurs before frame/serial/state mutation or send.
Transaction timeout	A read returns Timeout and the client is disconnected.	The one absolute TX-through-decode deadline expired. Reconnect explicitly before another operation; foreign responses and partial progress never extend the deadline.
Timeout configuration	setTimeoutMs() returns Busy, including when passed zero.	Timeout configuration is idle-only and Busy admission precedes value validation. Wait until the request ends or configure a separate client; the active request keeps its original deadline.
PLC error correlation	A non-zero PLC end-code response becomes ProtocolError, or a write becomes OutcomeUnknown with cause ProtocolError.	A structured error-information block did not match the active route, command, or subcommand. Reconnect because the transport generation is closed, inspect state before any state-changing retry, and use lastResponseFrame() for the complete rejected frame.
Unknown state-changing outcome	A write, remote-control, password, error-clear, or monitor-registration operation returns OutcomeUnknown.	Some request bytes may have reached the PLC, but confirmation failed. Read lastOutcomeUnknownReason(), inspect the PLC/process state, and never resend automatically.
Local close	Closing an active read leaves Closed; closing after a state-changing request may have started leaves OutcomeUnknown with cause Closed.	Treat close as transport-generation retirement. Reconnect explicitly and inspect state before considering another write.
Header selection	Names such as slmp::highlevel::readTyped or slmp::highlevel::Poller are undefined.	Include slmp_high_level.h explicitly in any file that uses the high-level facade.
Shared client	A second operation returns Busy.	One client owns one in-flight request and must not be called concurrently from multiple threads/tasks. Use independent transports and buffers for independent clients.
Bit-in-word update	A writeBitInWord or address-form writeTyped call with .bit succeeds but another writer's bit change is lost.	Both entry points use the same explicit two-request read-modify-write and are not atomic. Use PLC-side logic or an application-level ownership/handshake scheme when coherence matters.
Device-range evidence aggregate	A live range catalog uses several reads on QCPU/LCPU/QnU/QnUDV profiles.	The complete probe decision plan is prevalidated and owns one synchronous client turn, but the evidence is non-atomic. Any non-PLC-NG intermediate error is returned and the caller's catalog remains unchanged.