The SIGM20 has 8 sensor inputs that can be connected via SK7. These inputs can be connected to detectors from many sources.

Digitrax supply a four input block detector, the BD4, that works well with the SIGM20. Up to two BD-4 units can be connected to provide 8 block detect sections. The BD4 operates by sensing the current flow to the rails. The track has to be isolated, into sections; the current to each section is provided through the BD4. When it senses a current drawn by a decoder, it signals the SIGM20.

Connecting the BD-4

The BD4 outputs are connected using its plug DS-1. This connects to the SIGM20 SK7. A 20 way ribbon cable from The SIGM20 can be simply split into two, to connect to two BD4 boards. It connects to the connector marked “DS1” on the BD4.

SIGM20 SK7 pin 1st BD4 DS1 pin 2nd BD4 DS1 pin
1 1  
3 3  
5 5  
7 7  
2,4,6&8 2,4,6&8  
9   1
11   3
13   5
15   7
10,12,14&16   2,4,6&8

A suitable cable can be purchased from Sig-naTrak® as part number “CP_SIGM20” here

Connecting the cables


Programming the SIGM20

The SIGM20 inputs need to be selected to generate sensor messages. The settings are normally programmed using the Locoanalyse application for a PC.

Form item


Sensor Board Number

Choose a number between 1 and 255. This must be different from the number assigned to any BDL168 or SE8c boards.

On-board inputs

Sets how inputs from the BD4 are used. Choose “Sensor numbers 1-8” or “sensor numbers 9-16” when connected to a BD-4