An ST to LC duplex patch cord is a hybrid single-mode cord with two ST bayonet connectors at one end and an LC duplex connector at the other, on 9/125 µm fibre and usually 2.0 mm cable. It links legacy ST panels to LC switch and transceiver ports without re-terminating the installed cable. Radix Fibrotech makes them to order in Pune.
ST to LC duplex cord specification
An ST to LC duplex patch cord carries two single-mode fibres from a legacy ST panel to a modern LC port. At the ST end the two fibres finish in two separate bayonet connectors. At the LC end they finish in one duplex clip. Radix makes each hybrid cord to order in Pune. The ST connector guide and the LC connector guide cover each connector in full.
| Parameter | ST to LC duplex cord | Note |
|---|---|---|
| Fibre | 9/125 µm single-mode, OS1 or OS2 (ITU-T G.652.D), or bend-insensitive G.657 | Match the installed plant; see the OS1 and OS2 section below |
| ST end | Two simplex ST connectors, 2.5 mm ferrule, bayonet twist-lock | No duplex clip; the legs are marked 1 and 2, or A and B |
| LC end | One duplex LC, 1.25 mm ferrule, push-pull latch | Polarity is changed inside the clip |
| Polish | UPC at each end unless stated | State APC if the panel or port is APC; do not mix polishes |
| Cable | 2.0 mm duplex zip; 1.6 or 3.0 mm on request | The two legs split a short way from the ST end |
| Jacket | LSZH or PVC, riser or plenum on request; armoured for harsh routes | Yellow is the usual single-mode jacket colour |
Why use a hybrid cord instead of changing the panel
Many campus, industrial and utility sites were cabled when ST was the common single-mode connector. The permanent cable ends on ST adapters, but new switches, media converters and SFP transceivers carry LC ports. There are three ways to bridge that gap.
| Approach | Work at the panel | When it fits |
|---|---|---|
| ST to LC hybrid cord | None. The installed cable and ST adapters stay as they are | Equipment is being upgraded but the building cabling is not |
| Replace the ST adapters with LC and re-terminate the cable | Cut off the ST connectors and fit LC connectors or splice on LC pigtails, then test again | The whole panel is being rebuilt and downtime is planned |
| Replace the panel with an LC panel and splice fibre pigtails | New panel, new splices, new records | A long-term upgrade where ST should be removed from the site |
The hybrid cord is the lightest option because it touches nothing that is already working. It also keeps the move reversible: unplug the cord and the old ST equipment can be reconnected.
Where ST to LC cords are used
Typical single-mode uses are:
- Campus backbones. Building-to-building cable terminated on ST in older equipment rooms, now connected to LC switches.
- Industrial and utility sites. Plant networks and substations with ST panels, joined to newer LC-port industrial switches.
- Broadcast and defence rooms. Fixed or temporary links where ST connectors are already on the cable. Radix also makes armoured duplex tactical cable terminated in LC, FC, SC or ST for harsh routes.
- Test labs. Bench equipment with ST ports joined to LC test gear.
Once on LC, the reach comes from the transceiver and the whole link, not the cord. Under IEEE 802.3, 1000BASE-LX works to 5 km over single-mode fibre and 10GBASE-LR to 10 km. An old ST backbone adds its own splices and connectors to that budget, so measure the loss of the existing link before choosing the optic.
ST bayonet and LC latch: handling differences
The ST connector has a 2.5 mm spring-loaded ferrule inside a round body. A key on the body lines up with a slot in the adapter. The connector is pushed in and turned a quarter turn to lock the bayonet. Turn the connector's outer ring, not the cable, and do not pull on the cable to remove it. An ST adapter needs finger room all round, so a crowded panel may be easier to work from the cord side than from the equipment side.
The LC connector has a 1.25 mm ferrule in a small body. It is pushed in until the latch clicks and released by pressing the latch lever. The two ferrule sizes matter for cleaning and inspection. An inspection scope needs the 2.5 mm tip for the ST end and the 1.25 mm tip for the LC end, and a cleaning pen is made for one size or the other. Check both ends of a hybrid cord before mating, using the right tool at each.
Polarity and leg marking on a hybrid duplex cord
An ST connector has no duplex clip, so the two ST legs are held together only by the cable zip and are easy to cross by accident. The cord therefore needs marking. Each ST leg carries a number or a letter that matches a position in the LC clip.
A-to-B is the default duplex convention under TIA-568: the fibre in position A at one end lands in position B at the other, so transmit meets receive. For a hybrid cord, state which ST leg goes to which LC position. For example: ST leg 1 to LC position B and ST leg 2 to LC position A. If the link does not come up, swap the two connectors in the LC clip, then correct the position markings so the labels match the wiring. A visual fault locator at the LC end shows which ST leg lights up, which confirms the mapping in a few seconds.
OS1 or OS2 fibre, and how the cord is tested
OS1 and OS2 are cabling-standard classes for single-mode installed cable. ISO/IEC 11801 allows up to 1.0 dB/km for OS1, which is tight-buffered indoor cable, and up to 0.4 dB/km for OS2. Both use 9/125 µm fibre of the G.652 family. On a cord of a few metres, this attenuation difference is negligible, so an OS2 cord serves either plant. The class matters for the loss budget of the installed link, not for the patch cord.
Single-mode cords are measured for insertion loss at 1310 and 1550 nm, with return loss, and the end faces are inspected against IEC 61300-3-35 criteria. Ask for the maximum insertion loss per connector and a report format in the quote. The report for a hybrid cord should name the ST end and the LC end separately. If the ST panel is multimode, the cord must be multimode as well: single-mode cord into 62.5/125 µm or 50/125 µm plant is a core mismatch. Wider guidance is in the single-mode patch cord page.
What to send us for a quote
Give one line per configuration, with:
- fibre: OS1 or OS2, and G.657 bend-insensitive if routing is tight
- polish at the ST end and the LC end
- cable diameter, jacket rating and colour
- length and tolerance
- leg mapping: which ST leg goes to which LC position
- label text for each ST leg and the LC clip
- test report format and quantity
A photo of the ST panel and the label on the LC port helps confirm fibre type and polish. Send the list through the request a quote page. Radix quotes against the specification and does not publish a price list.
Questions buyers ask
Can I plug an ST cord straight into an LC port?
No. The connectors have different ferrule sizes and different bodies. A hybrid ST to LC cord has an ST connector at one end for the panel and an LC connector at the other end for the port, so it needs no adapter or converter.
Is there such a thing as a duplex ST connector?
Not in the usual sense. ST connectors are single-fibre, so a duplex ST cord has two separate connectors on a two-fibre cable. The word duplex refers to the two fibres and the zip cable, not to a clip.
Should an ST to LC cord be OS1 or OS2?
The cord is 9/125 µm single-mode either way. A few metres of cord adds almost no attenuation, so order OS2 for OS1 or OS2 plant. The class matters for the installed link, not the cord.
Do I need APC at the ST end?
Most single-mode ST plant is UPC. If the panel or the port is marked APC, state it in the quote and match each end to the port it meets. Never mate APC with UPC.
Does a hybrid cord add loss compared with an LC to LC cord?
Each end is one mated connector pair, as with any cord. The loss depends on the connectors and the panel, not on the ST to LC mix. Ask for the maximum loss per connector in the quote.
Can I use an ST to LC cord on a multimode ST panel?
Only if the cord is multimode with the same core size as the installed fibre. This page covers single-mode 9/125 µm. Check the panel and the cable print before ordering.