What is an ST connector?
The ST connector, short for Straight Tip, is a round fiber optic connector with a spring-loaded 2.5 mm ferrule and a bayonet coupling. To connect it, you line up the key, push the connector onto the adapter and twist the outer ring a quarter turn until it locks. To remove it, you push in slightly, twist it back and pull. It became a standard choice for early fiber LANs and campus backbones, especially on multimode fiber.
How to recognize it
- A round coupling ring, often metal, with a bayonet slot, similar in feel to a coax BNC connector.
- A long, round ferrule sticking out of the front.
- Simplex design: each connector carries one fiber, so a link uses a pair of ST connectors, one for transmit and one for receive, often marked or color-coded.
A common memory aid is "stick and twist" for ST, versus "stick and click" for SC.
ST vs. SC vs. LC
| Connector | Shape | Coupling | Ferrule |
|---|---|---|---|
| ST (Straight Tip) | Round | Bayonet twist-lock | 2.5 mm |
| SC (Subscriber Connector) | Square | Push-pull, snaps in | 2.5 mm |
| LC (Lucent Connector) | Small, square | Push-pull with a latch | 1.25 mm |
A common mistake is calling the ST a push-pull connector. Push-pull describes the SC and the latching LC. The ST always needs that twist.
Where you'll still see it
New installs mostly use LC, and SC in many provider and fiber-to-the-home settings, because they're smaller, quicker to connect and easy to pair in duplex form. ST connectors remain on older multimode backbones, patch panels and media converters in buildings and campuses, and on some industrial equipment. When new equipment has to join an older ST link, a hybrid patch cord (ST on one end, LC or SC on the other) bridges the two.
For the CompTIA A+ exam
Core 1 (220-1201) objective 3.2 lists ST alongside SC and LC as the fiber connectors to identify. Picture questions usually hinge on shape and locking method: round with a bayonet twist is ST, square push-pull is SC, and the small latching connector is LC.