PLC data to a web dashboard — designing Modbus polling

Most requests to view PLC data on the web converge to "read via Modbus, put it in a DB, and graph it." The problem is designing the polling cycle and storage structure.

Setting the polling interval

1-second polling is usually excessive. It is better to first observe the cycle of value changes.

Buffering — to avoid losing data even when communication drops

• On PLC communication failure, retry and then store in a local buffer
• On DB failure, temporarily store to files (SQLite/CSV) and apply on recovery
• Set an upper buffer limit and discard oldest entries first — preventing unbounded growth

Storage structure

If you put all samples into a relational DB, it grows quickly. In practice, it is better to separate the latest value (current value table) and history (time series) per tag.

-- Keep one row per tag for current values
CREATE TABLE tag_current (
  tag_id TEXT PRIMARY KEY,
  value REAL, quality INTEGER, ts TEXT
);

-- A time index is required for history
CREATE INDEX idx_tag_ts ON tag_history(tag_id, ts);

The dashboard does not need to refresh every second. Separating the screen refresh cycle from the collection cycle greatly reduces the load.