MODICON PC-E984-685 (984-685) Slot Mount Controller CPU/PS

Product Overview

The Modicon PC-E984-685 (984-685) is a high-performance, slot-mount Programmable Logic Controller (PLC) that integrates a Central Processing Unit (CPU) and a Power Supply (PS) into a single module. Part of the legacy Modicon 984 series, the E984-685 is engineered for mid-to-large scale industrial automation tasks requiring robust logic processing and high-speed I/O communication. This "All-in-One" architecture simplifies system planning by providing internal power for the CPU and the backplane while supporting the standard Modicon 800-Series I/O. Its dual AC/DC capability (115/220 VAC input and 24 VDC output) makes it a versatile solution for heritage systems in manufacturing and process control.

Technical Configuration (Deep Dive)

The PC-E984-685 architecture is designed for deterministic execution and high-density memory management.

• Integrated Power Supply: The module features a built-in power supply that accepts 115 VAC or 220 VAC (jumper/switch selectable). It provides the necessary 24 VDC and logic voltages required to power the CPU and up to a specific number of I/O modules on the local rack.

• Memory Management: Equipped with high-capacity user memory for logic (Relay Ladder Logic) and state RAM. It supports battery-backed CMOS RAM to ensure program retention during power loss.

• Dual Communication Ports: Typically features two Modbus (RS-232/RS-485) ports, enabling simultaneous connection to a programming terminal (like a P190 or PC-based software) and a human-machine interface (HMI) or SCADA system.

• Executive Firmware: The controller utilizes a swappable "Executive" cartridge or FLASH-based firmware that defines the instruction set and communication protocols, allowing for field updates to enhance functionality.

Technical Specifications

Technical FAQs

How do I clear a "CPU Stopped" error code on the E984-685?

A stopped CPU is often indicated by specific LED patterns (e.g., "Ready" LED off). Check the error codes using Modsoft or Concept software. Common causes include a depleted battery, memory parity errors, or an I/O mismatch in the configuration.

Can I use the 24 VDC output to power external field devices?

While the PC-E984-685 provides 24 VDC for internal rack logic and local 800-series modules, it is standard engineering practice to use a separate, dedicated external power supply for heavy field loads (solenoids, contactors) to prevent electrical noise from resetting the CPU.

What is the function of the "Config Mode" toggle switch?

The toggle switch allows the user to protect the memory (Run/Protect) or place the CPU into a configuration state for Modbus port address assignment and baud rate setup.

Engineering & Installation Guide

• Voltage Selection Jumper: CRITICAL: Before applying power, verify the internal voltage selection jumper is set correctly for your local mains (115 VAC vs. 220 VAC). Applying 220 VAC to a unit set for 115 VAC will result in immediate hardware failure of the power supply stage.

• Grounding and Noise Abatement: Ensure the PLC rack is grounded via a heavy-duty copper braid to a central "Star" ground point. Use shielded twisted-pair cables for Modbus communications to prevent electromagnetic interference (EMI) from high-power motor drives from corrupting the data stream.

• Battery Maintenance: Replace the lithium battery every 2-3 years while the unit is powered on. If the battery is removed while the unit is powered off, the user program and configuration will be lost.

Engineering Advantages

The PC-E984-685 is distinguished by its Instruction Set Stability. Unlike modern controllers that require frequent firmware patches, the 984-685 offers a "frozen" industrial logic environment that is highly predictable. Its integrated power supply reduces the physical footprint of the PLC system, making it an ideal choice for "Retrofit-in-Place" projects where enclosure space is limited but high-speed Modicon logic is still required.