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API Reference
Variables

Variables

Read and write PLC variables by symbol path. Requires feature flag Variables enabled.

Read Variable 

GET /api/plcs/{plcId}/variables/{symbolPath}

Policy: ReadAccess (role >= viewer, scope ads:read)

curl http://localhost:5000/api/plcs/Line1/variables/MAIN.nCounter
{
  "data": { "symbol": "MAIN.nCounter", "value": 42, "typeName": "UDINT" },
  "error": null
}

Batch Read 

GET /api/plcs/{plcId}/variables?paths=MAIN.a&paths=MAIN.b

Policy: ReadAccess

Reads multiple variables in one request. Maximum 100 symbols per request.

Write Variable 

PUT /api/plcs/{plcId}/variables/{symbolPath}

Policy: WriteAccess (role >= operator, scope ads:write)

curl -X PUT http://localhost:5000/api/plcs/Line1/variables/MAIN.nCounter \
  -H "Content-Type: application/json" \
  -d '{"value": 100}'

Batch Write 

PUT /api/plcs/{plcId}/variables

Policy: WriteAccess

curl -X PUT http://localhost:5000/api/plcs/Line1/variables \
  -H "Content-Type: application/json" \
  -d '{"values": {"MAIN.a": 1, "MAIN.b": 2}}'

Maximum 100 symbols per request.

Value Constraints 

GET /api/plcs/{plcId}/constraints/{symbolPath}

Policy: ReadAccess

Returns the configured validation constraints for a symbol, including Min, Max, Enum, Pattern, and ReadOnly fields. If no constraints are configured for the symbol, the response contains empty/null fields.

curl http://localhost:5000/api/plcs/Line1/constraints/MAIN.nSpeed
{
  "data": { "symbol": "MAIN.nSpeed", "min": 0, "max": 100, "enum": null, "pattern": null, "readOnly": false },
  "error": null
}

Writes are validated against these constraints. A write that violates a constraint returns 400 with error code VALUE_OUT_OF_RANGE.

Symbol Access Control

Write operations are checked against the symbol access guard configured per PLC (see Configuration). If a symbol is denied by the allowlist/denylist rules, the API returns 403 with error code SYMBOL_WRITE_DENIED.

PLCsSymbols