The Practical Guide to FDDI -- Physical Connections

FDDI has a sister, CDDI, which uses ordinary Cat5 twisted pair cable (the C stands for Copper) and RJ45 connectors. The pinout isn't the same as for Ethernet cables, but if you can make your own cables then you're set. CDDI is effectively identical to FDDI except for using copper; everything on the previous pages applies to CDDI as well.

FDDI Connectors

MIC connector The MIC connector, sometimes also called the FDDI connector, is common on concentrators and older cards. Its major problem is that it is physically large and many card formats, such as EISA or PCI or MCA, don't have enough space on the back panel for two connections. One MIC connector contains two fiber connections. MIC connectors are keyed in two different ways. First, they're keyed so you can't put them in upside down. Secondly, there is an optional key indicating what port type it is. In the concentrator photos, you can see the color-coded key inserts on the ports if you look closely.	A MIC female connector from the side. Note that the thing sticking out of the connector is just a dust cover. Looking down into the connector from the end, with the dust cover removed.
SC connector The SC connector is common on newer cards because it is smaller. Technically, the connectors shown here are dual SC, since each individual connector only contains one fiber, and two are needed for an FDDI connection. Dual SC connectors are small enough that two will fit on a PCI card back panel. It's easy to get confused between SC and ST connectors. I remember that SC are scware -- lame but effective. Another trick that's been reported is "C for Click, T for Twist". Dual SC connectors are keyed so you can't put them in upside down. You can see the keying slots on the ports and the matching tabs on the connector on the right.	A DAS card with two dual SC connectors, from the side. Looking down into the connectors from the end.
ST connector ST connectors are uncommon. Like SC, each individual connector only contains one fiber, and two are needed for an FDDI connection. Unlike SC, there is no such thing as a dual ST connector, so the male connectors are usually color-coded so that the Rx and Tx connections (labelled on the female connectors) can be crossed over from one end to the other (i.e. the fiber connected to Tx at one end obviously has to be connected to Rx at the other). ST connectors work similarly to BNC connectors: there is a tab on the connector which lines up with the slot on the port (visible on top in the photo), and then the port has two tabs (visible on the sides in the photo) which mate with a rotating sleeve on the connector. The sleeve twists, locking the connector onto the port.

Note that many FDDI cards have an "optical bypass" port. It's usually a DIN or an RJ45 connector (as in the photos of the SC card above). Do not confuse an RJ45 optical bypass port with a CDDI port. An optical bypass switch, which connects to the optical bypass port, is an optomechanical relay that physically modifies the light path to route around a failed or powered-down interface. It connects like this:

Also note that these connectors are used by things other than FDDI. ST connectors are most common on the various flavors of 10-megabit Ethernet which use fiber optics, such as 10baseFL. SC connectors are used for 100baseFX Fast Ethernet over fiber and for ATM over fiber.

FDDI Cabling

Putting connectors onto ordinary wire can be enough of a nuisance. It's much harder to do so for fiber optics, requiring special equipment to get perfectly polished surfaces. The best way to get FDDI cabling is to buy it premade. You can find a lot of it on Ebay.

Another caveat with fiber optics: be careful about bending them! They break more easily than wire. Also, too sharp of a corner can cause a cable to not work even without doing permanent damage. Fiber optics work by internal reflection: the light hits the walls of the fiber at a shallow angle and virtually all of it reflects back into the fiber. A sharp corner may increase the angle too much, decreasing the efficiency of the reflection and also causing the light to reflect more times as it goes down the fiber, diminishing below the detection threshold before it reaches the other end. That said, fiber optics aren't that scary, either. I have some which go through a bend about three inches in radius and they work fine.

CDDI Connectors

orange-white	brown-white
orange	brown
green-white	green-white
blue	blue
blue-white	blue-white
green	green
brown-white	orange-white
brown	orange

CDDI Cabling

<< BACK | NEXT >>

All text and pictures copyright (c) 2003 James Birdsall.