Each TMR digital input module has three sets of isolated electronic devices called channels, which independently process the module's field data. Each channel places the processed data in an array and transfers the array to the MP associated with the channel according to the request. MPs vote on data before passing it to the application. The DI module includes complete and continuous diagnostics for each channel. If the diagnostic program detects a fault on any channel, the fault indicator light will light up and activate the system alarm. The fault indicator identifies a channel fault, rather than an entire module fault. The DI module continuously verifies the system's ability to detect transitions to opposite states. The DI module ensures normal operation in the event of a single fault and can continue to operate normally in the event of multiple faults. The DI module supports hot standby modules. Each DI module is mechanically locked to prevent improper installation in the configured substrate. The 3301 type DI module can be used for the following substrates: 2301 type, suitable for typical applications. DI external terminal board, used for solid-state relay input external terminal panels in high-voltage applications, or DI hazardous area external terminal panels of type 9573-610.