984 AT4

This Modicon 984 model was a host-based controller built as an industry-standard ISA add-in PC card designed to reside in and execute control logic from a host computer.
The 984-AT4 communicates with I/O through a built in S908 compatible Remote I/O head/scanner.
They came with a ModbusPlus port as standard.
They were very useful for in-house development and testing of logic.

The family approach to 984 controller design allowed you to make choices based on controller capacity (the number of discrete and analog/register points available for application programming, the number of I/O drops it supports), throughput (the rate at which it solves logic and updates I/O modules), and environmental hardness.
The 984 instruction set (the functional capabilities of the controller, part of the system firmware stored in executive PROM) comprised logic functions common to other 984s. This means that user logic created on a midrange or high-performance unit such as a 984-685 or a 984B could be relocated to a smaller controller such as a 984-145 (assuming sufficient memory in the smaller machine) and that logic created on a smaller controller is upwardly compatible to a larger unit.

Programming was in ladder and we used a P190 programming panel or a PC to enter the program and for diagnostics. The 984 was a really advanced machine for it's time and was a great leap forward from the 584 / 884 families. They were often deployed widely and often served as master PLC's communicating to remote machines and HMI.
These used 800 series I/O, which are still in service in some locations.

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