PRODUCT INFORMATION
Product Overview
The Allen-Bradley 1785-L20B is a PLC-5 controller designed for industrial automation architectures. It provides 16 Kilowords of internal static RAM backed by a lithium battery. The unit delivers control across local and distributed remote input and output configurations. A dedicated 1770-XYC battery preserves volatile memory during routine power interruptions or shutdowns. Users can expand non-volatile storage using optional 1785-ME series EEPROM memory backup modules.
The controller features multiple embedded communication channels for networking and diagnostics. Channel 1A operates as a dedicated Data Highway Plus port for peer communication. Channel 1B supports configuration as either Data Highway Plus or a Remote I/O link. Channel 0 provides an optically isolated serial port supporting RS-232, RS-422A, and RS-423 standards. Front-panel status indicators show processor state, communication activity, battery health, and active forces. A front key-switch provides selectable Run, Program, and Remote operating modes.
TECHNICAL DATA
Technical Specifications
| Parameter | Specification |
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| UPC | 612598123971 |
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| Weight | 3.00 lbs (1.36 kg) |
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| Non-Operating Temperature | IEC 60068-2-1 (Test Ab, Un-packaged Nonoperating Cold), IEC 60068-2-2 (Test Bc, Un-packaged Nonoperating Dry Heat), IEC 60068-2-14 (Test Na, Un- packaged Nonoperating Thermal Shock): –40...85 o C (– 40...185 o F) |
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| Operating Temperature | EC 60068-2-1 (Test Ad, Operating Cold), IEC 60068-2-2 (Test Bd, Operating Dry Heat), IEC 60068-2-14 (Test Nb, Operating Thermal Shock): 0...60 o C (32...140 o F) |
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| Remote I/O | 12 chassis |
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| Maximum Number of I/O Racks | 4 (0-3) |
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| Battery | 1770-XYC |
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| Memory | 16 Kwords |
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| Embedded Communication interfaces | 1 DH+ (Fixed) 1 DH+/Remote I/O (Adapter or Scanner) 1 serial port, configurable for RS-232 and 423 and RS-422A standards |
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| Backplane current load | 2.3 A |
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| Power Dissipation | 12 W |
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| Power Supply | 1771-P4S, -P6S, -P4S1, -P6S1 |
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| Module Type | Standard Processor |
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| Processor | PLC-5/20 |
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| Max User Memory Words | 16 Kilowords |
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| Battery-Backed Static RAM | Yes |
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| EEPROM Program Backup Option (using 1785-ME16, -ME32, -ME64, and -M100) | Yes |
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| Protected Memory | No |
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| Max Total I/O | 512 (any mix) or 512 in + 512 out (complementary) |
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| Max Analog I/O | 512 |
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| I/O Scan Time Per Rack | 10 ms at 57.6 kbits/s, 7 ms at 115.2 kbits/s, 3 ms at 230.4 mbits/s |
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| Max I/O Chassis Extended Local | 0 |
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| Max I/O Chassis Total | 13 |
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| Max I/O Chassis Universal Remote | 12 |
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| RS-232-C/422-A/423- A Ports | 1 |
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| Control Coprocessor Expansion Port | Yes |
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TECHNICAL INFORMATION
Features
- Embedded memory capacity of 16 Kilowords of battery-backed static RAM.
- Supports optional 1785-ME16, 1785-ME32, 1785-ME64, and 1785-ME100 EEPROM backup modules.
- Dedicated Data Highway Plus communication interface available on Channel 1A.
- Configurable Channel 1B operating as Data Highway Plus or Remote I/O scanner or adapter.
- Optically isolated Channel 0 serial port supporting RS-232, RS-422A, and RS-423 standards.
- Controls up to 512 total I/O points and 512 analog I/O points.
- Front key-switch with Run, Program, and Remote mode selections.
- Diagnostic status indicators for processor condition, battery status, forces, and communication.
TECHNICAL INFORMATION
Connections / Interfaces
- Channel 0 features a 25-pin connector for optically isolated serial communications.
- Channel 0 supports ASCII as well as DF1 full-duplex and half-duplex protocols.
- Channel 1A uses a built-in port for dedicated Data Highway Plus network connections.
- An 8-pin mini-DIN connector on Channel 1A functions as a programming terminal port.
- Channel 1B configures as a Remote I/O scanner, Remote I/O adapter, or DH+ port.
- Includes a control coprocessor expansion port for compatible system expansion hardware.
TECHNICAL INFORMATION
Installation Guidelines
- Install the unit into a compatible 1771 I/O chassis slot.
- Power the chassis using compatible 1771-P4S, 1771-P6S, 1771-P4S1, or 1771-P6S1 power supplies.
- Verify the backplane power supply can support the 2.3 Amp current load at 5 Volts.
- Maintain operating ambient temperatures between 0 and 60 degrees Celsius.
- Install the 1770-XYC lithium battery to prevent loss of volatile static RAM data.
TECHNICAL INFORMATION
Configuration Guidelines
- Configure Channel 1B as a Remote I/O scanner to manage external remote I/O chassis.
- Set Channel 1B as an adapter to operate under a supervisory controller link.
- Ensure all devices on a DH+ network share identical transmission baud rates.
- Supported DH+ communication speeds include 57.6, 115.2, and 230.4 kilobits per second.
- Use the three-position key switch to select Run, Program, or Remote operating status.
- Switching to Remote mode allows software-driven program modifications during system operation.
TECHNICAL INFORMATION
Compatibility
- Operates within Allen-Bradley 1771 I/O chassis architectures.
- Compatible with 1785-ME16, 1785-ME32, 1785-ME64, and 1785-ME100 memory backup modules.
- Accepts power from 1771-P4S, 1771-P6S, 1771-P4S1, and 1771-P6S1 power supply units.
- Supports up to 12 universal remote I/O chassis on remote links.
- Does not support connection to extended local I/O chassis.
TECHNICAL INFORMATION
Lifecycle / Replacement Information
- The 1785-L20B belongs to the legacy Allen-Bradley PLC-5 control platform.
- Procurement availability is primarily through surplus, refurbished, and existing reserve inventory channels.
TECHNICAL INFORMATION
Compliance and Certifications
- Complies with IEC 60068-2-1, IEC 60068-2-2, and IEC 60068-2-14 environmental testing standards.
TECHNICAL RESOURCES
Documentation
COMPATIBILITY
Compatibility & Replacement
COMPATIBLE MODELSOperates within Allen-Bradley 1771 I/O chassis architectures. Compatible with 1785-ME16, 1785-ME32, 1785-ME64, and 1785-ME100 memory backup modules. Accepts power from 1771-P4S, 1771-P6S, 1771-P4S1, and 1771-P6S1 power supply units. Supports up to 12 universal remote I/O chassis on remote links. Does not support connection to extended local I/O chassis.
Verify compatibility against the exact Part Number, revision and manufacturer documentation before ordering.
PART VERIFICATION
Before You Order
Industrial automation parts can share similar model names, suffixes and revisions. Confirm the complete manufacturer Part Number before purchase.
01Check Part NumberMatch the complete ordering code on the existing unit.
02Confirm RevisionCheck suffix, hardware or firmware when applicable.
03Send a Label PhotoWe can review a clear label photo when identification is uncertain.
ORDERING PROCESS
From RFQ to Dispatch
1Submit Part Number2Verify Product Identity3Confirm Stock & Condition4Issue Quotation5Dispatch & Tracking
PRODUCT FAQ
Frequently Asked Questions
What type of memory does the Allen-Bradley 1785-L20B controller utilize?
The 1785-L20B contains 16 Kilowords of battery-backed static RAM. A 1770-XYC lithium battery maintains volatile data. Users can also install optional 1785-ME series EEPROM backup modules.
What communication ports are available on the 1785-L20B?
Channel 1A provides a dedicated Data Highway Plus port and mini-DIN terminal. Channel 1B configures for DH+ or Remote I/O scanner and adapter modes. Channel 0 offers an optically isolated serial port.
How many I/O chassis can the 1785-L20B controller support?
The 1785-L20B supports up to 13 total I/O chassis. This includes up to 12 universal remote I/O chassis. The unit does not support extended local I/O chassis.
What is the backplane current draw for the 1785-L20B?
The controller draws 2.3 Amperes of backplane current at 5 Volts DC. Power is supplied through compatible 1771 power supply units.
