An ST connector is a single-fibre optical connector with a 2.5 mm ceramic ferrule and a spring-loaded bayonet coupling that locks with a push and a quarter turn. It is defined in IEC 61754-2 and is used mostly on multimode fibre in older campus networks, industrial control, security systems and test equipment. A key on the body holds the ferrule at a fixed rotation.
How the ST connector works
The ST connector is a single-fibre plug with a 2.5 mm ferrule, usually zirconia ceramic, held in a spring-loaded body inside a metal or plastic bayonet nut. To mate it, you push the plug into the adapter and turn the nut a quarter turn until it locks over two studs. To release it, you turn the nut back and pull. The interface is defined in IEC 61754-2 and, in North America, in TIA-604-2 (FOCIS 2).
The body carries a key that slides into a slot in the adapter. The key sets the rotational position of the ferrule, and because only the nut turns, the ferrule goes in straight and does not twist against the mating end face. A spring then holds the two ferrules in physical contact. ST connectors are normally polished PC or UPC. Single-mode versions exist, but most installed ST plugs are multimode, on 62.5/125 micron (OM1) or 50/125 micron fibre.
ST is simplex by design. A duplex link uses two separate plugs, each mated on its own, and the bayonet needs finger room to turn, so ST panels hold fewer ports than SC or LC panels of the same size. A plug pushed in but not turned to the stop can sit loosely and show high or unstable loss, which is a common fault on site.
Where ST connectors are still used
ST was introduced by AT&T in the 1980s and became the usual multimode connector in campus and building networks before SC and then LC took over. Many of those links still work, and equipment with ST ports stays in service for a long time in some sectors. The ST fiber connector is still found on:
- Older campus and building backbones: patch panels and wall boxes from the 1990s, often on OM1 fibre, and early fibre Ethernet such as 10BASE-FL.
- Industrial Ethernet and control: media converters, PLC fibre modules and protection relays in utilities and process plants.
- Security and CCTV: fibre video transmitters and receivers, and alarm and access-control links between buildings.
- Legacy test equipment: older light sources, power meters and OTDRs fitted with ST ports or interchangeable ST adapters.
In these places the connector is set by the installed port, not by preference. New designs rarely start with ST, but replacement cords, extensions and repairs have to match what is on the wall.
ST vs SC vs LC
| Feature | ST | SC | LC |
|---|---|---|---|
| Ferrule | 2.5 mm | 2.5 mm | 1.25 mm |
| Coupling | Bayonet, push and quarter turn | Push-pull snap latch | Push-pull latch with lever |
| Duplex form | Two separate simplex plugs | Two plugs in a duplex clip | Two plugs in a duplex clip, standard on transceivers |
| Usual fibre | Mostly multimode | Single-mode and multimode | Single-mode and multimode |
| Common polish | PC or UPC | UPC and APC | UPC and APC |
| Panel density | Low | Medium | High |
| Interface standard | IEC 61754-2 | IEC 61754-4 | IEC 61754-20 |
| Typical use today | Legacy campus, industrial and security links | Telecom, FTTH and PON, older enterprise panels | Data centres, enterprise networks, SFP and QSFP ports |
ST vs SC: both use a 2.5 mm ferrule and give similar loss when well made. The difference is the coupling. SC snaps in with a straight push, needs no turning room and is widely offered in APC form, so it replaced ST in most new building and telecom work. Because the ferrules are the same size, ST to SC hybrid adapters exist, though a hybrid cord is usually the cleaner fix.
ST vs LC: LC has half the ferrule diameter and a duplex latch, so it fits at least twice as many fibres in a panel and matches the ports on modern transceivers. Linking ST plant to LC equipment needs a hybrid patch cord. Our LC connector guide and SC connector guide cover those plugs in detail, and the fibre connector types overview compares every family.
FC connector: the threaded alternative
The FC connector also uses a 2.5 mm ferrule and a keyed body, but it couples with a threaded nut screwed onto the adapter. It is defined in IEC 61754-13 and TIA-604-4 (FOCIS 4). The thread holds the plug firmly under vibration, and the key returns the ferrule to the same rotation at every mating, which matters for angle-polished and polarisation-maintaining fibre.
FC is common on test and measurement instruments, laser sources and sensing systems, and on some single-mode telecom equipment, where it was once a standard choice. It is offered in UPC and APC. FC/APC plugs and adapters come in narrow-key and wide-key versions, so state which one the equipment uses. Compared with ST, FC is the better fit for single-mode and APC work, while ST remains mainly a multimode connector. Both are slow to mate in a dense panel. For the polish question in general, see UPC vs APC.
ST connectors on rugged and armoured cable
The bayonet nut holds well against a tug, and many tactical and industrial systems were built with ST ports, so ST still appears on rugged cable. In those assemblies the cable does the protective work. An armoured or tactical jacket resists crushing, rodents and vehicle traffic, while the connector remains a standard physical-contact plug that needs caps and cleaning.
Radix makes duplex armoured tactical cable, and a 6-fibre version, terminated in LC, FC, SC or ST. When ordering an ST-terminated rugged assembly, state whether the plugs should be staggered and protected by a pulling grip for installation, and specify dust caps on every connector. For links that are mated repeatedly in dirt or water, a connector designed for field use may serve better than ST.
Hybrid cords: moving ST links to LC equipment
When a switch or media converter with LC ports replaces older equipment, the simplest step is usually an ST to LC duplex hybrid cord between the existing ST panel and the new port. Four points decide whether that cord works.
- Match the fibre core. Older ST plant is often 62.5/125 micron OM1. A 50 micron cord on 62.5 micron plant adds a large extra loss, often a few dB, in the direction from the larger core into the smaller one. Order the cord on the same core size as the installed fibre.
- Mark the polarity. The two ST legs are separate plugs. Label them A and B, or transmit and receive, so they map correctly to the LC duplex clip.
- Match the polish. ST plant is almost always PC or UPC. Keep UPC at both ends unless the equipment states APC.
- Check the link length. Newer, faster optics often have shorter reach on OM1 than the equipment they replace. Measure the link and check the optic's data sheet before ordering.
Where a panel has many ST ports in daily use, replacing the adapter plates and terminating the fibres in LC can be tidier than a drawer of hybrid cords.
What to specify for ST connector assemblies
- Connector at each end: ST, or ST to LC, SC or FC for a hybrid cord.
- Fibre type and core: OM1 (62.5/125), OM2, OM3 or OM4 (50/125), or OS2 single-mode.
- Polish: UPC as standard, APC only where the equipment calls for it.
- Simplex or duplex, and how the duplex legs are labelled.
- Cable construction: zip-cord, round, armoured or tactical, with the jacket rating your site requires.
- Length connector to connector, with the tolerance you accept.
- Maximum insertion loss per connector, a test report for every fibre, and dust caps.
Radix builds ST, FC and hybrid assemblies to order in Pune, India. Send the list above through the request a quote page.
Questions buyers ask
What does ST stand for on a fibre connector?
ST is usually expanded as straight tip. The connector was introduced by AT&T in the 1980s and is now defined internationally in IEC 61754-2. It uses a 2.5 mm ferrule and a bayonet coupling.
Is the ST connector single-mode or multimode?
Both exist, but most ST connectors in service are multimode, on 62.5 micron OM1 or 50 micron fibre. Single-mode links usually use SC, LC or FC instead. Check the fibre type printed on the cable jacket before ordering replacement cords.
Can I connect an ST connector to an LC port?
Not directly. Use an ST to LC hybrid patch cord with the same fibre type and core size as the installed link, and label the ST legs so that transmit and receive cross correctly.
What is the difference between ST and SC connectors?
Both have a 2.5 mm ferrule. ST locks with a push and a quarter turn of a bayonet nut, while SC latches with a straight push and has a square body that clips into a duplex pair. SC is more common in new work and in APC form.
Are ST connectors obsolete?
ST is a legacy choice for new designs, but it is still standardised and widely installed. Industrial, utility and security equipment with ST ports stays in service for many years, so ST cords and repair assemblies are still needed.
Which holds better under vibration, ST or FC?
FC. Its threaded nut is less likely to work loose than a bayonet that was not turned fully home, and its key holds the ferrule rotation. That is why FC is common on test instruments, lasers and sensing systems.