The 6ES7902-3AC00-0AA0 is a 10-meter RS422 point-to-point connection cable with 15-pin D-Sub connectors—plug (male) at one end, socket (female) at the other. It extends the serial link between a SIMATIC S7/M7 CP 340, CP 341, or CP 440 communication processor and the partner device—an operator panel, barcode scanner, third-party controller, or modem—across a distance of roughly one cabinet row.

Which length should I pick—5, 10, or 50 meters?

The 6ES7902-3 series comes in three standard lengths, and the choice is purely about physical distance between the CP and the communication partner. At 5 meters (6ES7902-3AB00), both devices sit in the same cabinet or on adjacent panels—the cable routes through a single wire duct. At 10 meters (this cable), the partner is in the next cabinet down the row—same wall, different enclosure. At 50 meters (6ES7902-3AG00), the partner is at the far end of the machine or in a separate control room. RS422 is rated for up to 1,200 meters at low data rates, so all three lengths are well within the electrical specification—pick the one that physically reaches without leaving a coil of excess cable to manage.

15-pin versus 9-pin D-Sub—which one do I need?

The 15-pin D-Sub is specific to RS422 communication on CP 340/341/440 modules. The 9-pin D-Sub cables in the 6ES7902-1 and 6ES7902-2 series serve different electrical interfaces—RS232C and TTY (20 mA current loop), respectively. The connector is physically keyed: a 15-pin plug will not mate with a 9-pin port. Check your CP module's port labeling—if it says RS422, you need the 15-pin series.