Section 3: Interconnection Methods

Solid State Interface

The standard TxDOT railroad interface actively controls traffic signal operations during railroad preemption through its own microprocessor using multiple preemptors that are associated with the specific control plan. This manual defines such an interface as an active-style where the interface is responsible for processing all railroad inputs and directing the traffic signal controller. The preemptors shall be configured as shown in Figure 5-1 and described in Chapter 5, Section 4. The “static” configuration assigns each preemptor a specific railroad circuit and is the TxDOT standard. Some active-style interfaces use a dynamic preemptor assignment, which shuffles the preemptor assignment based on the railroad circuits requested.
Figure 5–1. Configuration Output Chart (Static)
Figure 5–1. Configuration Output Chart (Static)
Active style interfaces require several preemptors to operate. Most ATC traffic signal controllers have sufficient preemptors to accommodate these demands and Emergency Vehicle Preemption (EVP), if desired. However, depending on the traffic signal cabinet used, there may not be enough preemption terminals on the main panel to access all of the available preemptors in the controller. Preemptors 1-6 shall be reserved for railroad preemption. EVP detection/inputs may be combined, where feasible, and connected to lower priority preemption inputs. Alternatively, other inputs may be remapped for EVP.

Relay Interface Panel

This manual defines relay interface panels which only pass railroad preemption inputs through to the traffic signal controller, as passive-style interfaces. With passive-style interfaces, the traffic signal controller is responsible for processing all railroad inputs from the interface and managing the preemption operations. Off-the-shelf, passive-style interfaces are available, but can require complex logic processing in the traffic signal controller to provide the necessary preemption functionality. As a result of this, along with the possibility of equipment failure and potential human error, TxDOT does not allow passive-style railroad interface panels. The designer shall not specify use of a passive-style railroad interface panel without a design exception and verification that the interface panel can provide the output as described in Chapter 5, Section 4.