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  • Lifecycle status

    Discontinued / Obsolete by OEM
    Apter Power coordinates condition-verified sourcing and migration documentation for this discontinued fixed-plus-configurable-channel controller.

1785-L20B

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A fixed DH+ path distinguishes 1785-L20B from PLC-5 controllers whose control-network ports are fully configurable; Rockwell Automation lists this catalog as discontinued.
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Technical specifications for 1785-L20B

  • Manufacturer:
    Allen-Bradley
  • Part Number:
    1785-L20B
  • Product Family:
    PLC-5
  • Product Type:
    PLC-5/20 Enhanced Programmable Controller
  • Stock:
    1
  • Controller family:
    PLC-5/20 Enhanced Programmable Controller
  • Maximum user memory:
    16 K words
  • Total I/O maximum:
    512 any mix or 512 inputs + 512 outputs complementary
  • Communication ports:
    1 fixed DH+ channel, 1 DH+/Remote I/O channel, and 1 configurable serial port
  • Maximum rack addresses:
    4 rack addresses (0…3)
  • Maximum I/O chassis:
    13 total; 12 remote
  • Backplane current:
    2.3 A at 5 V DC
  • Operating temperature:
    0…60 °C
  • Storage temperature:
    -40…85 °C
  • Relative humidity:
    5…95%, noncondensing
  • Vibration:
    2 g at 10…500 Hz
  • Operating shock:
    30 g
  • Non-operating shock:
    50 g
  • Enclosure type:
    Open style
  • Battery:
    1770-XYC
  • Chassis location:
    Left-most slot in supported 1771 chassis
  • Model-specific boundary:
    one fixed DH+ channel is separate from the configurable DH+/Remote I/O channel
  • Weight:
    1.45 kg
  • Tariff Code:
    8537101190
  • Country of origin:
    USA
  • Place of shipment:
    Xiamen,Fujian,China
By purchasing this item, you agree to the Terms & Conditions, ApterPower is not an authorized distributor for listed manufacturers or tradenames and therefore the manufacturer's warranty does not apply. All our products come with ApterPower's 1-year warranty.

More Information

Overview Manuals Fixed DH+ Path Thirteen-Chassis Plan Retirement Control Product Applications

Features:

  • Split communication design: one fixed DH+ channel plus one configurable DH+/Remote I/O channel
  • PLC-5/20 capacity: 16 K-word memory and up to thirteen I/O chassis
  • Exact lifecycle evidence: Rockwell Automation discontinued the catalog effective December 31, 2014
1785-L20B supports 1771 systems that need a permanent DH+ path while reserving another channel for DH+ or Remote I/O work.

This fixed-versus-configurable split is central to replacement acceptance because moving cables between the two ports changes the available network role.

A sustainable plan combines channel maps, rack addresses, project and EEPROM records, current firmware, and an engineering-replacement strategy.
Apter Power can support 1785-L20B quotation and procurement planning with official catalog and configuration-document review.
Allen-Bradley 1785-L20B PLC-5/20 Enhanced Programmable Controller Manuals, Datasheets, Links
Important Notice: Other accessories, manuals, cables, calibration data, software, etc. are not included with this equipment unless listed in the above stock item description. All prices are shown in USD.
  • Permanent role: keep the fixed DH+ channel on the documented programming or peer network rather than assigning it as Remote I/O.
  • Second channel: reserve the configurable DH+/Remote I/O port for the selected scanner, adapter, or additional DH+ duty.
  • Cutover map: record both cables and node settings so restoration does not silently swap channel functions.
  • Rack addresses: reconcile addresses 0…3 with the physical adapters and the controller project.
  • Remote scope: remain within twelve remote and thirteen total chassis, with no extended-local chassis allocation.
  • Link capacity: validate baud rate, termination, device count, and cable length independently of the logical chassis maximum.
  • OEM status: treat December 31, 2014 as the exact catalog discontinuation date.
  • Spare acceptance: verify catalog, Series/Revision, memory state, port face, and condition before relying on secondary-market hardware.
  • Engineering replacement: preserve the original rack and message inventory before evaluating a migration path.
  • PLC-5/20 1771 system control: execute an application within the 16 K-word and 512-I/O any-mix limits.
  • Concurrent DH+ and Remote I/O operation: retain the fixed DH+ route while assigning the configurable channel to the required I/O role.
  • Lifecycle-managed installed-base support: restore or migrate a discontinued controller using verified project, rack, and channel records.
Description
Rockwell Automation lists 1785-L20B as discontinued effective December 31, 2014 and identifies its replacement path as an engineering decision. Installed-base work therefore begins with a verified application archive and network inventory, then separates recoverable service requirements from functions that must be addressed in a migration plan.

The communication layout of 1785-L20B preserves one fixed DH+ path beside a second channel that can be configured for DH+ or Remote I/O. A configurable serial interface adds a separate communication route, so the front-port identity and saved channel assignment both matter when matching a replacement processor.

For 1771 I/O control, 1785-L20B provides 16 K words of user memory and supports 512 I/O in any mix, or 512 inputs with 512 outputs in complementary mode. Rack addresses 0 through 3 can cover thirteen total chassis, including twelve remote chassis, without an extended-local allocation.
Revisions

Revision Part #

Also known as

1785-L20B Series A
1785-L20B Series A, Revision R.2
1785-L20B Series C
1785-L20B Series C, Revision U.2
1785-L20B Series D
1785-L20B Series D, Revision L.2
1785-L20B Series E
1785-L20B Series E, Revision K.2
Alternative Names
Allen-Bradley 1785-L20B Rockwell Automation 1785-L20B AB 1785-L20B 1785L20B 1785 L20B PLC-5/20 Enhanced Programmable Controller 1785-L20B
Technical notes
The fixed DH+ port on 1785-L20B cannot assume the configurable DH+/Remote I/O role. Before loading a recovered project, identify both physical connectors, trace the installed cables to their intended networks, and compare the saved channel configuration with the four-address rack map; stop if a cable swap has changed the required network function.
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FAQS

The following is a list of questions commonly asked by our customers. Please contact us if you have a question about our process that is not answered below.
Why can Remote I/O fail after cables are swapped between the two network ports on 1785-L20B?
Identify the fixed DH+ port, restore the Remote I/O cable to the configurable DH+/Remote I/O channel, verify the scan list and baud rate, and then retest rack communication.
How should a thirteen-chassis layout be validated for 1785-L20B?
Reconcile rack addresses 0…3, the twelve remote chassis entries, physical adapter switches, per-link device count, termination, and cable length; reject any chassis that exists only in a copied scan list.
What changes in the sourcing decision because 1785-L20B is discontinued?
Treat December 31, 2014 as the OEM discontinuation date, verify secondary-market condition and Series/Revision, preserve the working project and network inventory, and evaluate the Engineering Replacement path before a failure forces conversion.
Why can an EEPROM that fits physically be unsuitable for 1785-L20B?
Check the memory-module catalog, processor Series/Revision, programming-software release condition, stored-project identity, and capacity before inserting or restoring it.
What should be investigated when the farthest remote chassis on 1785-L20B drops out at a higher baud rate?
Measure the actual trunk length, confirm every device uses the same rate, verify the required end resistors and supported device count, inspect dropline lengths, and reduce the rate only after the topology is documented.
How can a buyer verify 1785-L20B before a replacement controller is dispatched?
Require clear full-catalog, Series/Revision, front-port, battery, and memory-module photographs plus the project, rack map, condition, quantity, and destination; reject Ethernet or ControlNet siblings with similar PLC-5/20 naming.

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