ATL Telecom AM2048 - Manual (Page 45)

ascom
telecom USER GUIDE AM768A/AM2048A
45
Issue 8
Appendix A - X.21/V.35 Cable lengths
There is an inherent design limitation with the X.21 and V.35 interfaces when timing information is
only transmitted in one direction.
The timing is usually sent from the DCE to the DTE. Data originating from the DCE and travelling to
the DTE arrives at the DTE with the same clock skew with which it originated at the DCE. However,
data originating at the DTE arrives back at the DCE skewed relative to the DCE clock. This skew is
equal to (2 x cable delay) + Driver delays at both ends.
Receive data is normally sampled at the half bit period position. So to guarantee correct operation,
the data arriving at the DCE must avoid this half period delay.
NB. This calculation is only valid If the DTE samples receive data at the half bit position.
Example calculation
:
Note: The driver delays for the DTE are only ESTIMATED values.
For N = 32
Bit period = 488ns
Half period = 244ns
Typical V11 driver delay = 30ns
Total driver delay = driver delay x 4 = 120ns
Typical logic delay = 8ns
Typical cable delay per metre = 6ns
1st problem cable delay
= (Half Bit Period – total driver delays – logic delay)/2
= (244 – 120 – 8 /2
= 58ns
1st problem cable length = 9.7m
2nd problem cable length = (one and a Half Bit Period – total driver delays – logic delay)/12
= (732 – 120 - 8)/12
= 50.3m
3rd problem cable length = (two and a Half Bit Period – total driver delays – logic delay)/12
= (1220 – 120 – 8 )/12
= 91m
And so on…
So, further problem lengths occur at additional
(Bit Period)/(2 x Cable delay per metre) = 40.6m
See the next pages for the potential problem cable lengths for all values of N.
If a problem is experienced, try adding two metres to the cable length.
The latest versions of the X.21 and V.35 interface have features which help to overcome these
problems.