BECKHOFF EtherCAT Technology Section I - Manual (Page 73)

Distributed Clocks
Slave Controller – Technology I-53
Parameters used for propagation delay calculation are listed in Table 26:
Table 26: Parameters for Propagation Delay Calculation
Parameter Description
t
Px
Processing delay of slave x (through EtherCAT Processing Unit, x=A-F)
t
Fx
Forwarding delay of slave x (alongside EtherCAT Processing Unit, x=A-F)
t
xy
Propagation delay from slave x to slave y (x/y=A-F)
t
Wxy
Wire propagation delay between slaves x and y (assumed to be symmetrical in both
directions, x/y=A-F)
t
x0
, t
x1
, t
x2
Receive Time Port 0/1/2 values of slave x (time when first preamble bit is detected,
x=A-F), measured with a write access to DC Receive Time 0 register.
t
P
Processing delay (through EtherCAT Processing Unit) if all slaves are identical
t
F
Forwarding delay (alongside EtherCAT Processing Unit) if all slaves are identical
t
Diff
Difference between Processing delay and forwarding delay t
Diff
= t
P
– t
F
if all slaves are
identical. ESC specific information, part of the ESI. Refer to Section III for actual
figures.
t
Ref_x
Propagation delay from Reference Clock (slave A) to slave x
Propagation delay between Slave C and D
The propagation delays between slave C and D (t
CD
and t
DC
) consist of a processing delay and the
wire delay:
t
CD
= t
PC
+ t
WCD
t
DC
= t
PD
+ t
WCD
Assuming that the processing delays of slave C and D are identical (t
P
= t
PC
= t
PD
):
t
CD
= t
DC
= t
P
+ t
WCD
The two Receive Times of slave C have the following relation:
t
C1
= t
C0
+ t
CD
+ t
DC
So the propagation delays between slave C and D are
t
CD
= t
DC
= (t
C1
– t
C0
) / 2
Propagation delay between Slave B and C
The propagation delays between slave B and C (t
BC
and t
CB
) are calculated as follows:
t
BC
= t
PB
+ t
WBC
t
CB
= t
FC
+ t
WBC
Assuming that the processing delays of slaves B, C and D are identical (t
P
= t
PB
= t
PC
= t
PD
), and the
difference between forwarding and processing delay of slave C is t
Diff
= t
PC
– t
FC
:
t
BC
= t
P
+ t
WBC
t
CB
= t
BC
– t
Diff
The Receive Times (port 0 and 1) of slave B have the following relation:
t
B1
= t
B0
+ t
BC
+ t
CD
+ t
DC
+ t
CB
So the propagation delay between slave B and C is
2*t
BC
– t
Diff
= (t
B1
– t
B0
) – (t
C1
– t
C0
)
t
BC
= ((t
B1
– t
B0
) – (t
C1
– t
C0
) + t
Diff
) / 2