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scsi bus ringing/signal shape change
Hello,
Electronics newbie with a question. I have a DAQ card which is communicating with 8 perif boards over a SCSI bus. The communication is controlled by a 20Mhz TTL clock over one of the bus lines. The DAQ card has the option to have this clock internally generated by the DAQ card or by feeding it an external clock. For my application it will be necessary to feed the DAQ an external clock. I have noted the following: 1) When looking at the internally generated clock over the bus on a scope it looks perfect, 0-5V TTL. 2) When looking at my externally generated clock over the bus on the scope (the exteral clock also supplies a TTL 20Mhz signal with the same available power as the DAQ clock) the clock signal no longer looks squarish but rather deformed looking with a sort of ringing/extra occilating like behavior. As a note, the external clock looks fine on the scope when not fed onto the bus. Does anyone have any ideas as to why the external would look strange like this on the bus while the DAQ supplied clock looks fine on the bus even though they can supply the same amount of power? As a last note the SCSI bus consists of 9 cables: 1 cable that has 9 connector taps ~1 inch spacing, 1 tap for the daq and 8 taps for the additional 8 cables which are each 6 inches long and feed the individual perif boards. Thanks for any suggestions on this, Kev |
scsi bus ringing/signal shape change
You probably need a termination. Rise times in nanoseconds means harmonics
in the hundreds of MHz, easily enough to generate reflections and standing waves on unterminated lines. And woe is he who has a crappy line...I can't imagine ribbon cable is all that happy if you don't ground every other wire. Tim -- Deep Fryer: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms wrote in message ps.com... Hello, Electronics newbie with a question. I have a DAQ card which is communicating with 8 perif boards over a SCSI bus. The communication is controlled by a 20Mhz TTL clock over one of the bus lines. The DAQ card has the option to have this clock internally generated by the DAQ card or by feeding it an external clock. For my application it will be necessary to feed the DAQ an external clock. I have noted the following: 1) When looking at the internally generated clock over the bus on a scope it looks perfect, 0-5V TTL. 2) When looking at my externally generated clock over the bus on the scope (the exteral clock also supplies a TTL 20Mhz signal with the same available power as the DAQ clock) the clock signal no longer looks squarish but rather deformed looking with a sort of ringing/extra occilating like behavior. As a note, the external clock looks fine on the scope when not fed onto the bus. Does anyone have any ideas as to why the external would look strange like this on the bus while the DAQ supplied clock looks fine on the bus even though they can supply the same amount of power? As a last note the SCSI bus consists of 9 cables: 1 cable that has 9 connector taps ~1 inch spacing, 1 tap for the daq and 8 taps for the additional 8 cables which are each 6 inches long and feed the individual perif boards. Thanks for any suggestions on this, Kev |
scsi bus ringing/signal shape change
Tim Williams wrote: You probably need a termination. Rise times in nanoseconds means harmonics in the hundreds of MHz, easily enough to generate reflections and standing waves on unterminated lines. And woe is he who has a crappy line...I can't imagine ribbon cable is all that happy if you don't ground every other wire. Tim The DAQ card and perif boards all have termination. Would I need some diode termination setup at the external clock source too? Or is having it at the terminal devices sufficient? |
scsi bus ringing/signal shape change
In article , Don Bruder
writes SCSI is NOT voodoo. There are sound scientific reasons why there must be *EXACTLY* three terminations involved in any successful SCSI chain: The Termination at the Host machine, the Termination at the last peripheral on the chain, and of course, the Termination of the chicken Goats work better than chickens, but for those particularly intractable problems, it's best to go straight to a young virgin. -- (\__/) (='.'=) This is Bunny. Copy and paste Bunny into your (")_(") signature to help him gain world domination. http://www.thisisbunny.com/ |
scsi bus ringing/signal shape change
On Mon, 28 Aug 2006, Mike Tomlinson wrote: Goats work better than chickens, but for those particularly intractable problems, it's best to go straight to a young virgin. I've found that for those particularly intractable problems, it's best to go straight to your GP... Cheers, Pete. Ducking for cover and running away very, very fast |
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