984 Chassis Mount
Models - B, A, X

These Modicon 984 models were large, rugged, high-performance chassis mount controllers with no local I/O.
They came in three basic models, 984B, 984A, 984X.
They were top of the range from 1986 through 1994, when the Quantum series became available.

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.

Hot Standby capability was available, as an option, for applications that demanded fault-tolerant, high-availability performance. Two identically configured 984 controllers communicate with each other through two Hot Standby option modules, one in each controller. Each controller has the HSBY loadable software function block installed in the first segment of ladder logic.
The S911 and R911 Hot Standby modules were installed in option slots with their host controllers. The S911 was for slot mounts and the R911 for chassis mounts.

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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