Bell Interface Board

Introduction

The bell interface boards are DIN-rail mountable modules which can connect a computer to four independent bell lines. They are compatible with bell line voltages from 3 volts up to 24 volts, and connect via WiFi or Modbus over RS485. The modules inputs are protected by optoisolators and the outputs are driven by resilient ProFET electronic switches, which can handle indefinite short circuits. They can be programmed and updated via a microUSB connector on the side of the interface, or over WiFi.

They were developed to provide a high speed interface for block bells, where the Modbus IO boards we normally recommend for signal box interfacing proved too slow (10 Hz refresh rate). Instead, these boards use a dedicated microcontroller to sample the bell lines much faster, by default at 100 Hz. We challenge you to tap faster than this interface can handle!

An additional benefit to putting a dedicated microcontroller on the bell interface is that it can perform the parsing of bell codes locally. The interface sends and receives three types of information:

• Live tap information. Data packets are sent whenever the tapper is pressed or released. Useful for local networks where you can hear the other bell so any delays are annoying.
• Buffered tap timing. Once the code has been detected as complete, the relative timings for the taps are sent as a list and played back accordingly.
• Parsed codes. The bell interface parses the taps into a bell code, which is sent as a list of tap numbers.

For the second two information types, it is also possible to provide to the interface a code it is currently expected to receive. This allows the interface to reduce the delay before deciding the tapping has finished, or handle the case where a new code is sent immediately after a reply is given (the interface subtracts the expected code from the beginning and checks that the remainder is valid).

Status

• September 2024: In testing, firmware being finalised.

Files

• TBC

Suppliers

• TBC

Photos