Technical information

43
Installation
Performance Code A
Applications using cables with a performance code “A” will see degradation in positional
accuracy, increase in electrical noise susceptibility and possible instability.
The following graph shows a typical degradation curve of positional accuracy. Positional
accuracy degrades from ±15 arc-min at 15 feet to ±40 arc-min at 100 feet and ±70 arc-min at
200 feet. This must be considered when using cables longer than 100 feet.
As a general rule, electrical noise susceptibility of a cable increases with cable length. One
reason for this effect is the longer the cable the less effective the shielding. Also, the longer
the cable the more likely it will be exposed to an external noise source. There are several other
reasons, but the main point is long cable lengths are not recommended in electrically noisy
environments.
The final performance issue of code “A” is instability. During product design qualification
each FX drive is tested for stability with a load whose inertia is 10 times that of the inertia of
the motor on cable lengths up to 50 foot. An FX drive passes this test if it is stable at maximum
speed with a 10:1 load to motor inertia mismatch and a 50 foot cable. Cable lengths above 50
feet are tested but the drives may not always be stable at maximum speed.
Figure 30: Positional Accuracy vs. Cable Length
Performance Code B
Cables with performance code “B” are all used for serial communications. It should be noted
that the “Allowable” and “Maximum” lengths are both 50 feet. This limitation is a result of
the following statement from the EIA RS-232C specification:
“The use of short cables (each less than approximately 50 feet or 15 meters) is recommended;
however, longer cables are permissible, provided that the resulting load capacitance measured
at the interface point and including the signal terminator does not exceed 2500 picofarads
(pf).”
In FX drives the signal terminator capacitance is approximately 1000 pf and a typical cable
capacitance runs about 30 pf/ft. Therefore, the cable length should be limited to 50 feet.
Longer serial interface cables are not recommended.
CABLE LENGTH
POSITIONAL
ACCURACY
(ARC-MINUTES)
*
25 50 75 100 125 150 175 2000
10
70
60
50
40
30
20
*
*
*