Commissioning
Infrastructure for EtherCAT/Ethernet
40
Version: 2.1
Measurement
Explanations
Wiremap
Continuity test of all connected cores 1-8, screen
If, for example, a 4-core cable is measured, but an 8-core cable is specified in the
device, the wiremap test and thus all subsequent tests will fail.
Resistance
DC resistance/
loop resistance, given in Ω/100 m
normal: 12 Ω/100m @AWG22, 19 Ω/100m @AWG26
Length
Normally measured via NVP, which must therefore be entered correctly in the cable
data for the test.
NVP
(Normal Velocity of Propagation): ratio of the signal propagation speed in the
cable to the speed of light; usually 60 – 80% and to be taken from the cable data
sheet. Mainly results from the ‘length of lay’ and degree of twisting, e.g. where 2 m of
Ethernet cable contain 2+x m stranded wires per core. The larger the NVP value, the
less the cable elements are twisted.
The cable length as such is not actually a critical value according to EN50173, but
leads via length-dependent characteristic values (such as attenuation) to electrical
problems or via propagation-delay-dependent processes to protocol problems.
Propagation Delay
Results from the propagation delay of the signal in the cable. Leads to problems if a
permanent link measurement (specified at max. 90 m » usually 498 ns) is to be
performed on a 100 m Ethernet cable.
Differential signal
delay
Time delay in the signal propagation delay of a core pair. Should be 0 ns if possible.
Insertion Loss
Attenuation
The
parameter for the evaluation of the
cable
characteristics:
• the attenuation reduces the signal amplitude per meter of cable
• the attenuation is given as a positive value in [dB/100m] – SMALLER values
are better here
• the attenuation is frequency-dependent: the higher the frequency, the higher
the (real) attenuation is in the cable. As a result, the originally square signal
from the transmitter is smoothed to the well-known ‘eye’ shape – the receiver
must recover the signal by the use of equalizers
• an attenuation of 3 dB corresponds to a power loss of approx. 50%
• the attenuation increases
- if the cable becomes thinner (AWG number increases)
- if the cable is shielded (parasitic capacitances)
- if stranded cores are used instead of rigid cores
EN50173 permits different attenuation classes depending on the purpose
(permanently installed or device connection = patch cable), see Limit value records.
For orientation (according to EN50288-2:2003)
• Permanently installed cable: 21.3 dB/100m @ 100MHz (cable in stranded wire
execution for moving operation is also available according to this specification!)
• Patch cable/device connection: 32 dB/100m @ 100MHz
CAUTION: these are not the limit values according to which a complete cable
section is specified in accordance with EN50173!
Return Loss
Waves transmitted into the cable are partly reflected back to the transmitter by
defects. Defects may be in the material or at the plug transitions. The return loss is
the difference between the signal transmitted into the cable and the signal reflected
back.
• The higher the measured return loss the better; the attenuation is high and the
value (re-)received signal is thus smaller
• Order of magnitude: 10 dB/100m @ 100 MHz for the EN50173 Channel Class
D measurement.
