what is the difference between external SCSI cables and DAQ cables

[joelowwatt]

Hi all, I would like to make a simple extension using an external SCSI cable and a NI 184749/SH 6868. In other words that would be: HD68 female on bolth ends of the NI cable mated to an SCSI external cable with HD 68 male and a VHDCI 68 male. What are the differences between external SCSI cables and DAQ cables like the SHC 6868EP(M). Other than the gender of the HD68 end usually found on one. How/is the signaling affected buy using an external SCSI cable for an extension to say a BNC 2090?

 Yes I realize that NI wants us to buy their cables, however in these tough economic times one must make use of all available resources, so, try I must.

Thanks

joelowwatt  

[Seth_B. · Accepted solution]

Hello,

As long as you have compatible connectors, SCSI cables will work for basic DAQ functions.  They primary advantage of NI cables is that we can customize them to our DAQ devices.  As a result, we can create twisted pairs out of the AI differential pairs, using the ground terminals for shielding, and in general specialize the cable to minimize noise and cross talk.  Because SCSI cables aren't that specialized, you're likely to see more noise and, if you are going faster sampling rates, crosstalk.  If you are in a lab or factory with a fair amount of EMF noise your readings won't be quite as clean, or if you have florescent lights you might see 60Hz noise that you might not see otherwise.

If you'd like more details on the specifications of the SH68-68-EPM cable, I'd recommend these two articles.  The first demonstrates the twisted pair and shielding principles of the SH cable and the second gives more specific specs you can compare with when you are looking for an SCSI cable.

KnowledgeBase 3GRD4C33: SHC68-68-EPM & SH68-68-EPM Cable Wire Mapping List

KnowledgeBase 1TGEHPDM: Detailed Specifications for the SH68-68-EP Cable

 Regards,

[Jason_D]

joelowwatt,

I believe the major differences are in our signal integrity and shielding for data acquisition applications.  As it says in on the 68 page: "Features individually shielded analog twisted pairs for reduced crosstalk with high-speed boards." 

I really cannot tell you how your signal will be affected as this is very dependent on both your signal and the type and quality of cable you make.  As you can see here SCSI has many different types and this doesn't even start to take more of the physical characteristics into evaluation.  I would however guess that a main advantage between the 2090 and the setup you mentioned would be time spent and usability.

[joelowwatt]

I thank you bolth gentelmen, excellent advice. Once again NI has fit the bill, considering the value of my time I belive it would be better spent doing quality lab work than trying to save a couple bucks on a cable, so much for budget trimming 101. Glad I'm a scientist and not a bean counter.

 My best reguards

Joelowwatt