Festo CTEU-EC Serial Interface, PLC Expansion Module

What is the closest functional alternative to the CTEU-EC?

The closest peer is the CTEU-EP, which is also a serial interface module in the same CTEU series. The CTEU-EC and CTEU-EP share the same form factor and mounting, but the CTEU-EP weighs 0.04 kg versus the CTEU-EC's 0.10 kg — the difference likely reflects a different connector or internal electronics. Both modules serve the same role as a serial communication node for Festo valve terminals, so the CTEU-EP can drop into a panel specified for the CTEU-EC without rewiring, provided the controller protocol matches.