kumar@wucs1.wustl.edu (Arun Kumar) (12/10/88)
I would like to get in touch with people who are working to implement a communication link with the TAXI chip-set from AMD. I have built a transmitter/receiver pair, using TAXI's, that attempts to transmit a 512*512*8 picture from one frame buffer to another over an optical fiber. For electro-optic and opto-electric conversion I use the ODL200 chip set from AT&T. The circuit used to control the TAXI is built with FAST chips from Fairchild. The FAST logic is wire-wrapped, yet it appears to work as expected. The fast ECL connections between the TAXI's and the ODL200's are on a plain vector board (no ground plane), but are short as short can be. I will soon be putting everything on a 4-layer PCB. I seem to lose about 1 pixel per hundred at transmission speeds of about 70 million bps. I would like to run at the FDDI speed of 125 mbps. The TAXI chip set I have is not the long-promised final- version from AMD, and I suspect that therein lies all the problem. Is there anyone out there working with TAXI's, or implementing ANSI's FDDI standard, who can help or sympathize? --------------------------------------------------------------------- Arun Kumar kumar@wucs1.wustl.edu Box 1045, Computer Science (314)889-6160 Washington University, St. Louis, MO 63130
murray@jumbo.dec.com (Hal Murray) (12/13/88)
We have some in the lab. They work as expected, but we may not have pushed the rough edges. I haven't tried fibers yet. I think we only ran the old 70MHz versions at about 60. That was the crystal we had handy. We have a pair of the latest and greatest as of several weeks ago. They are still marked "SAMPLE", but work at full speed. Maybe the offical production ones are on the shelves now. What sort of error patterns are you seeing? The 4/5 encoding on the serial link means that a "simple 1 bit error" probably turns into a clump of 4 mashed bits, or 8 if the error crosses a nibble boundry. (That's assuming you are running in 8 bit mode.) If the fiber link is the problem, you should be able to see "funny" data going into the recv TAXI and/or VLTNs on the reveiver output. Sync the scope back on the transmit side and look at the eye pattern. I don't think there are any specs in the data sheet, but it has to be pretty crisp. More like a good digital signal rather than the sort of signal that a fancy modem would be happy with. AMD has a bunch of slides that talk about TAXI error rates. They claim 200 ft of RG-58A/U gets a byte error rate of 10^-10 at 70MHz. (I think this was with the 70MHz parts.) This is for 2 hunks of coax in parallel to carry the differential ECL signal. You could try cutting out the fiber gizmos. A twisted pair should work for several feet. Alternatively, try making the fiber path worse by adding some attenuation, say by making the fibers longer. If you have to insert a lot more fiber before anything changes, the fiber link is probably good enough.