Interface Cables Cha
ter Two: Installation
14
Generic Shielded Cable Description
MKS offers a full line of cables for all MKS equipment. Should you choose to manufacture your
own cables, follow the guidelines listed below:
1. The cable must have an overall metal
braided
shield, covering all wires. Neither
aluminum foil nor spiral shielding will be as effective; using either may nullify
regulatory compliance.
2. The connectors must have a metal case which has direct contact to the cable’s shield on
the whole circumference of the cable. The inductance of a flying lead or wire from the
shield to the connector will degrade the shield’s effectiveness. The shield should be
grounded to the connector before its internal wires exit.
3. With very few exceptions, the connector(s) must make good contact to the device’s case
(ground). “Good contact” is about 0.01 ohms; and the ground should surround all wires.
Contact to ground at just one point may not suffice.
4. For shielded cables with flying leads at one or both ends; it is important at each such end,
to ground the shield
before
the wires exit. Make this ground with absolute minimum
length. (A ¼ inch piece of #22 wire may be undesirably long since it has approximately
5 nH of inductance, equivalent to 31 ohms at 1000 MHz). After picking up the braid’s
ground, keep wires and braid flat against the case. With very few exceptions, grounded
metal covers are not required over terminal strips. If one is required, it will be stated in
the Declaration of Conformity or in the instruction manual.
5. In selecting the appropriate type and wire size for cables, consider:
A. The voltage ratings;
B. The cumulative I
2
R heating of all the conductors (keep them safely cool);
C. The IR drop of the conductors, so that adequate power or signal voltage gets to the
device;
D. The capacitance and inductance of cables which are handling fast signals, (such as
data lines or stepper motor drive cables); and
E. That some cables may need internal shielding from specific wires to others; please
see the instruction manual for details regarding this matter.
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