EL1252 by Beckhoff

The EL1252 is a Beckhoff EtherCAT digital input module in the EL EtherCAT Terminals series. It inserts two timestamped input channels into the fieldbus, providing edge-accurate detection for automation tasks that require coordinated motion or logging. Each incoming signal is coupled with a hardware time derived from the EtherCAT Distributed Clocks, so the PLC can evaluate the exact order and latency of machine events. This capability makes the terminal useful for high-speed counting, cam switching, and time-critical interlocks on production lines.

During operation, the terminal captures edges with a granularity of 1 ns because that interval is the fundamental clock unit used for its internal timebase. The absolute time is stored in a 64-bit register, allowing overflow-free logging for many years after power-up. The module samples up to 2 digital inputs independently, and the Distributed Clocks hardware keeps their jitter to well below 1 µs, so events from different terminals remain correlated. For every transition, the module forwards the 0/1 state and timestamp, giving the controller both the signal level and its occurrence time. Selection logic lets the user decide whether the first or last edge in a cycle is retained. This choice is configured with the first- or last-edge setting.

Edge acquisition uses event-based sampling, so cyclic polling does not consume bandwidth when no change occurs. Within one controller cycle, the terminal can buffer one rising and one falling edge per channel; additional transitions are suppressed until the next bus update, limiting data volume while preserving sequence integrity. The stored timestamp uses the epoch 2000-01-01 00:00, allowing each nanosecond counter value to be converted directly into real-world time for diagnostic or trace functions without external arithmetic.