System and method for providing delayed start-up of an activity monitor in a distributed i/o system

Abstract

A modular distributed I/O system includes a computer coupled to module banks through a network bus. A module bank includes a communication module, terminal bases, and I/O modules. The adjoined terminal bases form a local bus mastered by the communication module. The I/O modules connect to the local bus through terminal bases. I/O modules are pluriform and programmable. The communication module maintains a memory image of the configuration state of each I/O module resident in the module bank. A memory image persists when an I/O module is removed from its terminal base. The memory image is used to configure a new I/O module which is inserted into the same terminal base. The communication module monitors for communication failure on the network bus, and is configured to capture the state of the module bank and automatically restore this captured state after a power-loss event. The terminal bases realize a local bus which includes a parallel bus, a serial bus, and an address assignment bus. Each terminal base receives a value from a preceding terminal base, increments this value, and asserts the increment value to a succeeding terminal base. Each terminal base is automatically assigned an physical proximity address. An I/O module controls read/write access to its register space by a semaphore mechanism which supports multi-threaded processing. The I/O module latches an ungranted semaphore request and assumes a self-exclusion state upon releasing the semaphore wherein the semaphore is reserved for the communication module.