The latest news from the Unipart Rail Group of Companies

Resolving SSI communications issues

Resolving SSI communications issues

Using Park Signalling Technology for reliable service

Tags: Improved Performance Infrastructure Park Signalling Value - Increased Digital Value - Increased Performance Value - Innovation Value - Reduced Risk

The in-service reliability performance of the SSI Long Distance Terminal (LDT) design has never met expectations, with the module failures often manifesting themselves as shutdowns of the LDT causing it to cease communication. The LDT is not fitted with a fuse (as is the case for the SSI trackside modules) and thus can be reset by the maintenance technician by powering on. This combined with the fact that the LDT forms part of a redundant network, i.e. duplicated for availability, means that most railway operators have tolerated the problem.

The addition of an external filter designed and manufactured by Park Signalling, significantly improves performance of the system. The LDT LF01 Filter fits in series with the SSI LDT and its associated external wiring.

The Mean Time Between Failure (MTBF), calculated using MIL-HDBK-217, is 31.5 Years. The module is of robust construction and is suitable for attachment to its associated LDT Module housed either in a Network Rail standard pattern SSI Equipment location case or SSI Interlocking Cubicle.

The LDT LF01 Filter has been designed to meet the requirements of BR 967 Category D (the same requirements as the SSI LDT Module). All electrical connections are made via 75-way CANNON Trident connectors. Pin allocation for the connector is defined in Appendix One of specification BR1906 (September 1989). Note that the N12/B12 battery connections are not through connected in the LDT LF01 Filter.

The LDT LF02 Filter has been developed in response to a problem found at some installations where fitting the LDT LF01 Filter caused missing telegrams on the SSI data link.

·       The LDT LF02 Filter is for use with the SSI Long Distance Terminal Type SS 2501 only and for installations using current loop 1.

·       The LDT LF02 Filter shall be used for all new installations requiring only current loop 1.

·       The LDT LF01 Filter shall be used for all new installations that require more than one current loop.

To watch a video on the LDTF02, click here.

For further information and support, please contact

Adding Value with Increased Digital, Increased Performance, Delivered Innovation and Reduced Risk.


About this author
Dave Tilmouth

Head of Marketing Communications

I'm Head of Marketing Communications for Unipart Rail, so I get to be involved in lots of exciting stuff across Traction & Rolling Stock, Infrastructure and road traffic. My colleagues know me as someone who has a perspective on everything, as well as being a bit of an extrovert!


Leave a Comment

Name (required):

Email (Will not be published) (required):

Your thoughts: (Characters remaining: ) )

SPAM Protection: Sum of 17 + 4 ?

Read the Ts & Cs? Tick to confirm you agree to abide by our house rules

Enter text shown in this image into the box below.

Search Our Blogs

Recent Tweets

Commenting Guidelines
By commenting on this blog you agree to abide by our Terms and Conditions. Although we will do our utmost to avoid it, we reserve the right to edit, move or delete any comments which do not follow the guidelines provided