Application Scenarios
Consider a distribution center where a CompactLogix-controlled conveyor system must read barcodes from incoming totes and print routing labels at each divert point. The barcode scanners are older RS-232 units with fixed 9600-baud serial outputs—no Ethernet, no USB, no modern fieldbus option. The label printers accept RS-232 command strings and expect a specific termination character. The 1769-ASCII sits in the CompactLogix chassis and gives the controller two independent serial ports: Channel 0 reads the scanner’s barcode string, Channel 1 sends the label data to the printer. No external protocol converter, no third-party gateway, no additional power supply. The controller’s ladder logic reads the incoming string from the module’s input tags, validates the barcode, and writes the formatted label command to the output tags. The module handles the serial timing, buffering, and handshaking, presenting the controller with clean data blocks over the backplane .
Parameter
| Main Parameters | Value / Description |
|---|---|
| Product Model | 1769-ASCII |
| Manufacturer | Allen-Bradley (Rockwell Automation) |
| Product Category | Dual-Channel ASCII Serial Interface Module |
| Compatible System | CompactLogix (1769 I/O platform) |
| Serial Standards | RS-232, RS-485, RS-422 (per channel, configurable) |
| Channels | 2 independent serial ports |
| Data Buffer Limit | 200 bytes combined (Simultaneous mode); 200 bytes per channel (Alternating mode) |
| Baud Rate | Configurable via Studio 5000 (common rates: 9600, 19200, 38400, 115200) |
| Character Configuration | Data bits, parity, stop bits configurable per channel |
| Backplane Current | 425 mA @ 5.1V DC |
| Module Width | 1 slot |
| RPI Range | 1.0–750.0 ms (0.5 ms increments) |
| Operating Modes | Alternating (one channel at a time) or Simultaneous (both channels) |
| Handshake Modes | Immediate, Master/Slave |
Parameter notes: The 200-byte combined buffer limit in Simultaneous mode is a critical constraint—if both channels must operate concurrently with large strings, the sum of their data cannot exceed this ceiling . The backplane draw of 425 mA at 5.1V is substantial for a single-slot module and must be accounted for when power-budgeting a populated CompactLogix chassis . The module supports byte-swap modes for 16- or 32-bit boundaries, useful when interfacing with devices that transmit data in a different endian order .
Technical Principles and Innovative Values
Innovation Point 1: Two independent serial channels in one CompactLogix slot. Many CompactLogix applications need to talk to more than one serial device—a scanner and a printer, a scale and a legacy drive. The 1769-ASCII provides two configurable channels without consuming two slots or adding external gateways. Each channel can be configured independently for baud rate, parity, and protocol, so a 9600-baud scanner on Channel 0 can coexist with a 19200-baud printer on Channel 1. In Alternating mode, the controller services one channel at a time; in Simultaneous mode, both channels operate concurrently within the 200-byte buffer limit .
Innovation Point 2: Backplane-paged data blocks that integrate cleanly with ladder logic. The 1769-ASCII does not expose raw serial registers to the controller. Instead, it pages data between its internal buffers and the controller’s input/output tags in structured blocks with headers, transaction IDs, and status bytes. The controller’s ladder logic reads received strings from input tags and writes transmit strings to output tags, with transaction ID handshaking to confirm data freshness. This architecture means the controller’s scan time is not consumed by character-by-character serial polling—the module handles the timing, and the controller sees complete data blocks at the configured RPI .
Innovation Point 3: Delimiter-based and fixed-length receive modes for protocol flexibility. The 1769-ASCII can be configured to receive strings terminated by a specific delimiter (such as carriage return or line feed) or to receive a fixed number of characters. This matters when integrating devices with different message conventions: a barcode scanner may send a CR-terminated string, while a weigh scale may send a fixed 12-character frame with no delimiter. The module also supports a receive timeout value in 1 ms increments—if the delimiter does not arrive within the configured window, the module flags a non-delimited condition and presents the partial data to the controller, preventing a communication stall from blocking the logic .
