George Urchuk
August 16th, 2004, 12:59 PM
Hello,
I'm a Project Manager for a small process control team. We're currently working with a customer who is in the design stage for a mine that will use exclusively Foundation Fieldbus instruments and valves. Our customer is responsible for the segment design and has a question about different cable types and their effects on distance limitations.
The question relates specifically to Type A and Type B cables and the guidelines provided in table 6.1.1 of the Foundation Fieldbus Wiring and Installation Guide.
Type A is single twisted pair with shield.
Type B is multiple twisted pair with shield.
Our customer is looking at the problem from a 'first principles' perspective and is trying to understand why, if you follow the table, going from 1 pair (type A) to 2 pair (type B) will drop your distance allowance from 1900 to 1200 while if you go from 2 pair to 32 pair (I'm using 32 pair as an example since the table does not list an upper limit for the number of pairs.) there is no change in the distance limit.
My assumption is that this is be cause type A can be considered as 'without interference' while type B, be it 2, 4 or 128 pairs is considered to be 'with interference' and the number of pairs does not affect the amount of interference to a significant degree. However, I'm trained as a mechanical engineer so my grasp of electrical signals has never really progressed beyond thinking of them as water in a pipe and I can't provide a first principles answer.
As a result, I have two questions:
1) Is there any Foundation Fieldbus documentation available that approaches the above from a first principles or similar perspective?
2) Is my assumption that the number or pairs in a Type B cable is not significant beyond having 2 or more instead of one pair (Type A) correct?
Thank-you for any help that can be provided,
George Urchuk,
NORPAC Controls
I'm a Project Manager for a small process control team. We're currently working with a customer who is in the design stage for a mine that will use exclusively Foundation Fieldbus instruments and valves. Our customer is responsible for the segment design and has a question about different cable types and their effects on distance limitations.
The question relates specifically to Type A and Type B cables and the guidelines provided in table 6.1.1 of the Foundation Fieldbus Wiring and Installation Guide.
Type A is single twisted pair with shield.
Type B is multiple twisted pair with shield.
Our customer is looking at the problem from a 'first principles' perspective and is trying to understand why, if you follow the table, going from 1 pair (type A) to 2 pair (type B) will drop your distance allowance from 1900 to 1200 while if you go from 2 pair to 32 pair (I'm using 32 pair as an example since the table does not list an upper limit for the number of pairs.) there is no change in the distance limit.
My assumption is that this is be cause type A can be considered as 'without interference' while type B, be it 2, 4 or 128 pairs is considered to be 'with interference' and the number of pairs does not affect the amount of interference to a significant degree. However, I'm trained as a mechanical engineer so my grasp of electrical signals has never really progressed beyond thinking of them as water in a pipe and I can't provide a first principles answer.
As a result, I have two questions:
1) Is there any Foundation Fieldbus documentation available that approaches the above from a first principles or similar perspective?
2) Is my assumption that the number or pairs in a Type B cable is not significant beyond having 2 or more instead of one pair (Type A) correct?
Thank-you for any help that can be provided,
George Urchuk,
NORPAC Controls