When Serial Devices Refuse to Talk to Your CompactLogix
Barcode scanners, weigh scales, and legacy printers often sit on the plant floor speaking ASCII while your CompactLogix controller listens on a different protocol. The Allen-Bradley 1769-ASCII closes that gap. This specialty communication module gives your control system two independent serial channels, so you can bring ASCII-compatible equipment into the same automation architecture without adding a separate gateway PC.
What the 1769-ASCII Actually Does
The 1769-ASCII is an ASCII communication module from Allen-Bradley / Rockwell Automation. It installs directly in a 1769 CompactLogix chassis or beside a MicroLogix 1500 base unit. Two DB-9 male ports handle the serial traffic. Port 1 supports RS-232 and RS-422 full-duplex operation, while Port 2 supports RS-485 half-duplex. Each port configures independently, so one channel can poll a barcode reader while the other talks to a weight indicator.
Key Specs That Matter on the Floor
Two Ports, Two Protocols, One Module
The module provides 2 full-duplex inputs (RS-232, RS-422) and 2 half-duplex inputs (RS-485). You can run alternating mode or dual simultaneous mode. That flexibility means fewer part numbers on your shelf and simpler spares planning.
Power and Thermal Profile
The 1769-ASCII draws 425mA at 5Vdc from the backplane. Maximum power dissipation is 2.13 Watts, and thermal dissipation is 7.3 BTU/hr. Because it is backplane powered, you do not need an external supply — a real advantage in crowded panels.
Isolation and Signal Levels
The serial ports are isolated from the backplane with 30 Volts continuous isolation. Serial input voltage signals are 3…25V dc with respect to signal ground for active states, and -3…-25V dc for inactive states. This isolation protects your controller from ground loops and voltage spikes on long serial runs.
Distance Rating and Mounting
The CompactLogix power supply distance rating is 4. Install the module no more than four modules away from the power supply. Mounting options include chassis mount, DIN rail, or panel. Recommended cable is Belden 8761 shielded.
Where This Module Earns Its Keep
Barcode Reading on a Packaging Line
A scanner sends ASCII strings to the 1769-ASCII over RS-232. The CompactLogix controller reads the data and routes packages based on the decoded barcode. The second port can simultaneously drive a label printer, eliminating a separate serial card.
Weight Data from a Bench Scale
An RS-485 scale transmits weight readings to Port 2. The controller compares the reading to a setpoint and triggers a fill valve. Because the ports are isolated, noise from the valve solenoid does not corrupt the serial data.
Legacy Printer Integration
Many plants still run serial printers for work orders or shift reports. The 1769-ASCII lets you keep those printers without maintaining an old PC. The module handles the ASCII protocol directly, so your CompactLogix program controls the output.
Installation and Setup Essentials
Mount the module within four slots of the CompactLogix power supply. Use Belden 8761 shielded cable for serial runs. Configure each port for the correct protocol — RS-232, RS-422, or RS-485 — and set the baud rate to match the connected device. The module uses 31 configuration words and 108 input words. RTS/CTS hardware handshaking is enabled, which helps prevent data loss when the controller is busy. Verify that the serial port status LEDs show normal activity: TX FIFO overflow, RX FIFO overflow, RX parity error, handshake error, and new data flag indicators help you diagnose wiring or configuration problems quickly.
Maintenance and Long-Term Reliability
Inspect the DB-9 connectors for bent pins or corrosion during scheduled downtime. Check that the module sits firmly in its slot and that the DIN rail or panel mounting hardware remains tight. Keep the panel temperature within the 0-62 deg. C operating range. Because the module has no moving parts, wear is minimal. The main risk is environmental: dust and humidity. Store spare units between -40 and 85 deg. C in low-humidity conditions. For safety-related systems, follow the manufacturer’s safety manual and inspect periodically.
Compatibility and Integration
The 1769-ASCII works with 1769 CompactLogix controllers and the MicroLogix 1500. It communicates over the backplane, so no external power supply is needed. The module fits any adjacent 1769 Compact module. For MicroLogix systems, install it beside the base unit. The ASCII protocol support means it integrates with barcode readers, scales, printers, and other serial devices that follow the ASCII standard. Transaction IDs range from 0 to 255, which helps with message tracking in larger systems.
What to Verify Before You Buy
Confirm your controller is a 1769 CompactLogix or MicroLogix 1500. Check that your serial devices use RS-232, RS-422, or RS-485 and that you have the correct cables — Belden 8761 is recommended. Verify the power supply distance rating of 4 fits your chassis layout. If you need a spare, the 1769-ASCII is a current product. SpareTechPLC stocks this module and can help you confirm compatibility with your existing setup.
Frequently Asked Questions
Can I use both ports at the same time with different protocols?
Yes. Port 1 supports RS-232 and RS-422 full duplex, and Port 2 supports RS-485 half duplex. Each port configures independently, so you can run different protocols simultaneously.
Does the 1769-ASCII require an external power supply?
No. The module is backplane powered and draws 425mA at 5Vdc from the CompactLogix or MicroLogix backplane.
What cable should I use for serial connections?
Belden 8761 shielded cable is recommended for reliable serial communication.
How many modules can sit between the 1769-ASCII and the power supply?
The power supply distance rating is 4, so the module must be within four modules of the power supply.
Is the 1769-ASCII still available?
Yes, the 1769-ASCII is a current product. Contact SpareTechPLC for availability and lead time.
