Allen-Bradley 1769-ASCII | Dual Port ASCII Communication

Product Overview

The Allen-Bradley 1769-ASCII is a high-performance, dual-port serial communication module designed for the CompactLogix and MicroLogix 1500 platforms. This module enables seamless integration between your PLC and various ASCII-based devices, such as barcode scanners, weigh scales, serial printers, and legacy sensors. 

Technical Configuration & Hardware Architecture

The 1769-ASCII features a sophisticated hardware design that prioritizes signal purity and multi-protocol flexibility. It functions as an intelligent I/O module that resides directly on the 1769 backplane.

Dual Isolated Port Topology

Each port on the 1769-ASCII is electronically isolated to 30V continuous, protecting the backplane from ground loops and induced noise common in long-run serial cabling. Port 1 is optimized for RS-232 and RS-422 full-duplex communication, while Port 2 provides robust RS-485 multi-drop capabilities, allowing for versatile field device networking.

Backplane Integration

The module draws its 425mA operating current directly from the 5V DC backplane, simplifying enclosure wiring by eliminating the need for external power supplies. This hardware-level integration ensures that the communication ports are synchronized with the controller's internal clock for precise data time-stamping.

Technical Specifications

Installation & Engineering Guide

To ensure optimal performance of the 1769-ASCII, engineers must adhere to specific mechanical and electrical placement rules.

Power Supply Distance Rating

The 1769-ASCII has a power supply distance rating of 4. This means the module must be installed within four slots of the 1769 system power supply to ensure it receives consistent, regulated voltage for its dual-port circuitry.

Cabling and Grounding

For high-reliability data transmission, use high-quality shielded twisted-pair cabling (such as Belden 8761). Proper termination of the cable shield at the chassis ground is essential to minimize the impact of EMI on the ASCII string data, especially when operating at the maximum baud rate of 115.2 kbps.

Engineering Advantages

• Simultaneous Communication: Both ports operate independently, allowing the system to read from a barcode scanner on Port 1 while sending data to a printer on Port 2.
• Protocol Versatility: Native support for multiple RS-standards allows for easy retrofitting of legacy factory equipment into modern ControlLogix-based architectures.
• Rugged Industrial Design: Capable of operating in high-temperature environments (up to 60°C) without the need for active cooling, provided that the 50mm clearance rule is followed.

Technical FAQs

Q1: Does the 1769-ASCII support Modbus RTU?
A1: While the module is hardware-capable of RS-485, it is primarily designed for raw ASCII string communication. Implementing Modbus RTU requires manual ladder logic programming to handle the Modbus frame and CRC checks.

Q2: Can I mix RS-232 and RS-485 on the same module?
A2: Yes. The 1769-ASCII ports are independent. You can configure Port 1 for RS-232 communication with a PC and Port 2 for an RS-485 network of sensors simultaneously.

Q3: Is external power required for the serial signal?
A3: No. The module generates the necessary signal voltages internally from the 5V backplane power. However, the connected device must provide its own power source for its communication interface.