User guide

Hardware
12
890 USE 108 00 August 2001
The 984 Family of PLCs
Common 984
Controller
Architecture
Modicon’s Compact Programmable Controllers bring the high performance,
application flexibility, and programming compatibility of the 984 family to the small
controller market. As a controller in the 984 family, Compacts implement a common
instruction set for developing user logic, standard Modbus communication
functionality, and optional Modbus Plus communication capabilities.
The Compact Controllers share the following 984 processing architecture features:
A memory section that stores user logic, state RAM, and system overhead in
battery-backed CMOS RAM and holds the system’s Executive firmware in
nonvolatile PROM or FLASH RAM
A CPU section that solves the user logic program based on the current input
values in state RAM, then updates the output values in state RAM
An I/O processing section that directs the flow of signals from input modules to
state RAM and provides a path over which output signals from the CPU’s logic
solve are sent to the output modules
A communications section that provides one or more port interfaces. These
interfaces allow the controller to communicate with programming panels, host
computers, hand-held diagnostic tools and other master devices, as well as with
additional controllers and other nodes on a Modbus (or Modbus Plus) network
This architectural consistency allows the Compact Controllers to achieve machine
compatibility with the other controllers in the family. This allows sequences of user
logic created on a mid-range or high performance controller, such as a 984B or a
984-685 Controller,-to be relocated to a Compact. It also assures that user logic
generated for this small controller is upwardly compatible to larger 984(s). Thus, a
Compact can be easily integrated into a multi-controller network.