The Practical Guide to FDDI -- Port Types

FDDI ports come in four logical flavors:

A and B

Dual-attach stations have A and B ports. The classic dual ring is formed by connecting the A port of one machine to the B port of the next and so on until the A port of the last machine in the ring connects to the B port of the first machine, completing the ring.

M (master) and S (slave)

These are the ends of single-attach connections. The concentrator has M ports, and the stations have S ports.

Hooking them up

PORT	OFFICIALLY CONNECTS TO	WILL NORMALLY WORK CONNECTED TO
A	B	M or S
B	A or M	S
M	A, B, or S	--
S	M	another S

Hooking them up FAQ

Can I connect a dual-attach thing to a concentrator?

Yes! Just plug the cable from the concentrator's M port into the B port on the station, and it will work just fine. You can usually plug into the A port instead, and most concentrators will support a full DAS connection if you connect the A and B ports to two contiguous M ports on the concentrator.

All I have is dual-attach hardware. Am I set?

Yes, if you're willing to live with the limitations of the dual-ring topology. A to B, A to B, etc., and then you can either complete the ring or not. If you don't complete the ring, the machines on either end treat the missing connection in the same way as a failed connection and wrap. Depending on your hardware, this may cause warning lights to illuminate, but it's a legitimate configuration.

I only have single-attach hardware and no concentrator. Am I hosed?

Well, maybe. If you only have two stations, you can create a degenerate ring by hooking the slave ports together, as long as you make sure that the Rx and Tx connections are crossed over.

I have mostly DAS hardware, but I have a couple SAS stations as well. It looks like I should be able to just stick the SAS stations on the ends of the DAS chain, right?

This is reported to work.

Where can I find out more than I ever wanted to know?

Check out the rather comprehensive FDDI FAQ.

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