What it does on the rack
The Omron CS1W-CT021 is a Counter Module in the CS1W series, sitting on the CS Series backplane and presenting four digital inputs against four solid-state outputs to the CPU scan. Field wiring lands on a D-Sub connector at the module face, so the pulse sources — encoders, proximity counters, flow-meter pulses — terminate to a single multi-pin plug rather than screw terminals. It runs across a 0°C~55°C ambient window and carries CE, cULus, Lloyd, and NK approvals, which covers the usual UL-panel and marine-class builds without a separate submittal package.
Closest functional sibling
The other CS1W siblings on file (CS1W-SCU31-V1, CS1W-ETN01) are Communications Modules and are not cross-shop candidates for a counter slot.
Panel-side fit
D-Sub termination keeps the wiring job on a single plug, which is the right choice when the four input sources are encoder or pulse lines and you want one crimp-and-go harness rather than four discrete terminal landings.