ST Connector
Direct Answer
The ST connector (Straight Tip, TIA-604-2 / FOCIS-2) is a legacy fiber optic connector developed by AT&T Bell Laboratories. Featuring a 2.5 mm spring-loaded ceramic ferrule with a BNC-style twist-and-lock metal bayonet coupling, it remains common in industrial networks and campus multi-mode backbones.
Source: TIA-604-2-B (FOCIS-2) / IEC 61754-2
Specs
| Standard / FOCIS | TIA-604-2-B (FOCIS-2) / IEC 61754-2 |
| Ferrule Diameter | 2.5 mm |
| Ferrule Material | Zirconia ceramic or stainless alloy |
| Latching Mechanism | Bayonet twist-lock (BNC-style keying) |
| Fiber Modes Supported | Multi-Mode (OM1/OM2 primarily) and Single-Mode |
| Typical Insertion Loss | 0.25–0.40 dB |
| Polish Type | PC / UPC (Round metal housing) |
| Mating Cycles | 500+ cycles |
AT&T Legacy and Bayonet Lock
Introduced by AT&T in the late 1980s, the ST connector was the dominant multi-mode fiber connector throughout the 1990s in corporate local area networks (10BASE-FL and 100BASE-FX). Its keyed bayonet ring twists and locks securely into place over a slotted mating sleeve, preventing accidental dislodgement.
Limitations and Industrial Role
Because the ST connector requires rotational twisting to engage and disengage, it cannot be tightly packed on dense switch faceplates. While largely superseded by LC and SC in datacenter applications, ST remains widely used in industrial automation, power substations, and military field communications due to its mechanical ruggedness.