Echo in DLMs

[shekar413]

Hi,

What is the use of Echo in DLMs? Why is it disabled in Data link repeaters?

Also please confirm why Chassis Earth and Filter earths are used in DLMs?

[PJW]

Hi,

What is the use of Echo in DLMs? Why is it disabled in Data link repeaters?

Also please confirm why Chassis Earth and Filter earths are used in DLMs?

Straining my memory as have not dealt with this myself for best part of two decades!  Echo allows a technician trackside to use a Data Link Interrogator to see both the messages going to and coming from the central interlocking from monitor points connected to a spare (i.e. Not allocated to a TFM) output from DLM.  

I am guessing that if echo is enabled between a pair of back to back DLMs used as a repeater that there is the equivalent to acoustic howl as the output of one gets fed back via the other to be its own input and therefore messages on the datalink will get drowned out and the system collapses.

Therefore design should not provide monitor points on back to back repeaters since the technician won't see all the messages because echo has to be disabled.


I am not quite sure what you are asking in your second part.  Again I don't have manuals to hand or the time to look at them if I did, but imagine that the difference is the distinction between 
a) the low resistance earth for ensuring that the body of the DLM is kept at earth potential to ensure staff safety and to give some Faraday cage type screening for the electonics within,
and 
B) the Fast Transient low inductance earth that is required for lightening protection of the datalink connected to the Furse units or equivalent.

Does this make sense from looking at the standard drawings?

However there may be others on here who could help more- perhaps Rob from Park Signalling knows or could find out from an SSI expert.....?

[Jerry1237]

We have to be careful when talking about earthing. More often we mean bonding.

Common bonding is connecting conductive structures/elements together to protect personnel. Your A)!

Sometimes we tie conductive structures/elements down to mother earth as a way to remove large currents, generally in fault conditions. Your B) - maybe!

Sometimes we tie conductive structures/elements down to a conductor which acts as a return.

E&B is a large and complex subject, often misunderstood. E&P colleagues should be able to enlighten you further.