Rockwell Automation PLC-5 Fieldbus Solutions for Integrated Architecture User Manual - Manual (Page 241)

Publication 1757-UM006A-EN-P - May 2002
Fieldbus Wiring Considerations D-5
Cable Guidelines A quick method to determine if the segment is within limits is to
calculate a segment ratio. Total cable length is the sum of trunk and
spur lengths. If different cable preference types are used, apportion
each type against its limit and be sure the total is less than one (1).
Cable Attenuation Signals attenuate (or get smaller) as they travel though cables.
Attenuation is measured in decibels (dB) based on the following
formula:
dB = 20 log (V1/V2)
Where:
V1 = Amplitude of Signal Transmitted in Volts
V2 = Amplitude of Signal by device in Volts
• Cables have different attenuation ratings for a given frequency.
The frequency of interest for fieldbus is 39 kHz.
• The preferred type 1 cable for fieldbus has an attenuation of 3
dB/km.
• A fieldbus device can transmit a signal as low as 0.75 volts
peak-to-peak and detect a signal as small as 0.15 volts
peak-to-peak. This means that the cable can attenuate the signal
by 14 dB (20 log (0.75/0.15) = 14 dB).
Based on using the preferred type 1 cable a fieldbus cable run could
be up to 4.6 km (15091 ft.) long, as determined from this formula:
14 dB / 3 dB/km = 4.6 km
A shorter cable will have proportionately less attenuation. For
example, a preferred type 1 cable that is 500 m (1640 ft.) long would
have an attenuation of 1.5 dB.
EXAMPLE
For example, if the 500m (1640ft) trunk cable is
preferred type 1 cable and the spurs are 100m (328ft)
preferred type 2 cable, the total cable resistance
equals 0.35 (500/1900 + 100/1200 = 0.35).
0.35 is less than 1.0, therefore segment resistance is
good.