When relay interlocking equipment is connected to a computer interface, signal states must be identified unambiguously from feedback signals [1]. The indication displayed on SDO AW should represent the monitoring result received from the controlled object rather than merely indicate that a command has been sent. Distinguishing commands from indications forms the signal-related part of the closed command-execution-confirmation-indication loop.

For the entrance signal, the source specifies a lamp proving relay (O), a calling-on relay (PS), and red (K) and green (Ya) signals. For the exit signal, it specifies a lamp proving relay (O), a signal relay (S), and a shunting signal (MS) [1]-[3].

Here, Q_i is the set of states, Σ_i is the set of input combinations, and δ_i is the state update function. The model classifies current information as follows:

If the input combination remains unchanged, the state is retained. When the combination changes, the new state is determined from the table. Such a transition represents a change in monitoring information; it does not replace the algorithm that authorizes signal clearance. The interlocking safety logic is retained separately.

The combinations are denoted by X₀ - X₁₅. The index is determined from the binary value of the signal vector:

Table 1. Entrance signal input combinations and states.

The states are defined as follows: S₁, red; S₂, green; S₃, red and white; S₄, no information; and S₅, an inadmissible combination. The original designation of S₅ as “non-existent” does not mean that the signal is physically absent. In this paper, it is interpreted as a diagnostic state indicating an input combination not admitted by the model [4]. Table 1 provides the complete mapping.

Grouping the combinations in Table 1 yields an indicator function for each state. C_i = 1 means that the current combination belongs to state S_i. The symbols ¬, ∧ , and ∨ denote logical NOT, AND, and OR, respectively.

C₁ covers X₁ - X₇ and X₁₀; C₂ equals one only for X₉; C₃ for X₁₂ and X₁₄; C₄ for X₀ and X₈; and C₅ for X₁₁, X₁₃, and X₁₅. Each combination in the table belongs to exactly one state [5][6]:

Figure 1 illustrates the entrance signal state identification rule. The next state is determined by the current input group, independently of the previous state. Arrow colours identify the destination states and their associated input groups.

Figure 1. Mapping entrance signal input combinations to states.

For example, receiving X₉ = (1, 0, 0, 1) identifies S₂. If K changes from 0 to 1, X₁₁ = (1, 0, 1, 1) is obtained and the transition S₂ → S₅ is recorded. This represents a transition from the state designated as permissive to a diagnostic state. When Ya changes from 1 to 0, X₁₀ = (1, 0, 1, 0) is obtained and the automaton identifies S₁. These examples are table-based calculations rather than physical operational tests.

For the exit signal, the states are defined as S₁, red; S₂, white; S₃, extinguished; S₄, flashing white; and S₅, a non-existent state (Table 2).

Table 2. Mapping actually specified in the exit signal table.

The signal state is associated with an object identifier. A monitoring record may be structured as follows [7][8]:

Object_ID identifies the particular signal; Object_Type specifies its type; State contains the calculated state; Timestamp records the time of the monitoring information; and Quality indicates data quality. Here, R_sig is a monitoring record and is not equivalent to a control authorization in the interlocking system.

Information validity is assessed separately using v(t). It jointly represents communication availability, reception of signals within a consistent time interval, and compliance with the specified data freshness requirement. The freshness threshold must be established from the requirements of the actual system. The indication rule is written as follows:

The “unknown” indication denotes a communication or data quality problem. In contrast, S₄ in the entrance model is a table-defined state associated with particular zero-valued input combinations. Keeping these concepts separate prevents valid zero signals from being confused with communication loss. When S₅ is received, the combination, signal identifier, and timestamp are recorded in the diagnostic log [9]-[11].

The monitoring process comprises receiving signal feedback, associating it with the object, checking time and quality, identifying the state using the relevant table, distinguishing diagnostic states, and updating the indication and log. The outcome of a control command is assessed from confirmed feedback received from the signal. Transmission of a command does not establish that it has been executed.

An exhaustive combinational check was performed for the entrance signal: each of the 16 binary vectors was evaluated using Equations (5)-(9) and compared with Table 1. The results are summarized in Table 6. This calculation establishes internal model consistency; it does not measure the reliability or safety level of the actual equipment [8][9].

Table 6. Distribution of combinations in the entrance model.

The calculated combinations are mutually exclusive and cover the entire input domain. The distinction between functional aspects and monitoring codes is important: diagnostic state S₅ must not be interpreted as a fifth physical signal aspect. The functional model represents operational dependencies, whereas the decoder interprets feedback information. In the corrected entrance signal graph (Figure 1), the input group associated with S1 is X1 - X7 and X10; X12 belongs to S3.

The exit model table contains all eight combinations but covers only states S₁ and S₄. Thus, completeness of the combinations does not imply completeness of the state definitions. The practical outcome of the logical verification is a consistent rule for the entrance signal decoder and identification of the exit signal dependencies that require clarification.

Information relating to entrance, exit, and shunting signals was extracted, and separate mathematical expressions were formulated for entrance and exit signals. For the entrance signal, 16 combinations were mapped unambiguously to five states; completeness and mutual exclusivity of the state indicators were verified by combinational calculation.

The exit-table incompleteness is a substantive limitation of the supplied data and remains honestly stated. Resolving it requires the author-approved exit signal relay truth table; no unsupported new states or experimental claims were added.