984 Slot Mount
Series - 3, 4, 6, 7

These Modicon 984 models were rugged, midrange-performance slot mount controllers, which resided in a primary housing beside 800 Series I/O modules.
The housings came in two sizes, designed for 19" or 27" rack mounting.
They came in four basic models, (380,381,385),(480, 485), (680, 685), (780, 785).
The xx5 models had ModbusPlus.
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-perform ance 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.

Useful Links

984 System Manual
984 Hot Standby Manual
984 and 800 Series I/O