The Allen-Bradley 1785-L20B is a Programmable Logic Controller (PLC) module. This product is a PLC controller that belongs to the legacy PLC5 product family.

The 1785-L20B features a variety of embedded communication ports such as One (1) DH+ (Fixed), One (1) DH+/Remote I/O (Adapter or Scanner) One (1) serial port, configurable for RS-232 and 423 and RS-422A standards. These communications ports support individual configuration and independently functions from each other, They can be used simultaneously to implement the intended functions such as communication to compatible partner devices for data exchanges, communication with visualization devices and interaction with programming workstation.

The 1785-L20B’s embedded memory amounts to 16Kwords. This memory is a volatile memory, maintained by a lithium battery with catalog number 1770-XYC. This controller supports implementation of distributed I/O architecture via the DH+ or Remote I/O protocol. It may also be optionally installed with an Ethernet interface module for implementation of distributed I/O system over Ethernet. Up to Twelve (12) remote I/O chassis may be installed with this controller.

The 1785-L20B Controller supports installation of a Electronically Erasable Programmable Read Only Memory (EEPROM) as memory backup device. Memory options include 1785-ME16 with 16Kwords, 1785-ME32 with 32Kwords, 1785-ME64 with 64 Kwords and 1785-ME100 with 100Kwords.



The 1785-L20B has one built-in port dedicated for Data Highway Plus (DH+) communication interface marked as channel 1A. It also has One (1) combo port that can be configured both as DH+ or Remote I/O (adapter or scanner) marked as channel 1B. DH+ communication can be used both for remote programming and monitoring, creating peer-to-peer communication network between other PLC-5 controllers and communication with visualization systems. Remote I/O connection is used for real time data exchange between the controller and an I/O operator interface and other devices. Additionally, the 1785-L20B controller has a 25-pin connector labeled as channel 0 which is an optically isolated EIA RS232C port. This port supports ASCII and DF1 full-duplex, half-duplex master and half-duplex slave protocols. The controller’s other communication port includes an 8-pin mini DIN connector marked as channel 1A. which functions as DH+ programming terminal with parallel connection to channel 1A.

Channel 1B on 1785-L20B can be configured as remote I/O scanner or remote I/O adapter communication. The remote I/O scanner mode is used to read and write I/O information between a controller and remote I/O device. When in this mode, 1785-L20B is a supervisory controller for other controllers that are in adapter mode and remote I/O adapter modules. When channel 1B is configured as remote I/O adapter mode, the controller monitors and control its controller-resident local I/O while communicating with the supervisory controller via a remote link. On the 1785-L20B, the Data Highway Plus (DH+) link can be used on channel 1A (this channel is dedicated to this type of communication), and on channel 1B. The maximum cable length for DH+ link depends on the used transmission rate. It is crucial that all devices on a DH+ link are configured to communicate at the same transmission rate. That length goes from a maximum of 3048 m for a speed of 57.6 kilobits per second to 762 m for a speed of 230.4 kilobits per second.

The 1785-L20B has several indicators on the front panel used for easy interaction with the user. Indicators are used for displaying various statuses of the controller. Indicator marked BATT is used for displaying the status of the battery (red when the battery is low). Indicator marked PROC indicates controller current mode, it is green when the controller is in RUN mode, and red when some fault exists. FORCE indicator is colored amber when I/O forces are enabled. COMM indicator is linked with channel 0 communication status. The indicator is green when the channel is communicating. Channel 1A and 1B both have a dedicated status indicator.

A Three-position key-switch is provided for selecting the controller mode. Each position corresponds to one mode of the processor. RUN mode is used for running controller program, outputs are enabled. In RUN mode, I/O can be forced and data table values can be edited. Programs can be saved. In program mode (PROG position of key switch) outputs are disabled, and ladder files, SFC files or data files can be created, modified and deleted. It is possible to download and upload transfers with a memory module. Programs can be saved and restored. The third mode of the 1785-L20B processor work is remote mode, which is selected by moving the key switch to REM position. In this mode, it is possible to choose between three sub-modes: remote program, remote test, and remote run. When the processor is in remote run sub-mode, outputs are enabled and programs can be saved, restored, and edited while operating. Remote program sub-mode is same as normal program mode. While in remote test sub-mode, ladder programs can be executed with outputs disabled, ladder programs or data files cannot be created or deleted. Programs can be saved, restored and edited while operating.

The 1785-L20B has 16000 words user memory space. The controller’s RAM has a battery backup. For program backups, an EEPROM module can be installed in the slot situated on the front panel. Memory modules with various capacities are available. The total number of I/O (including local I/O and universal remote I/O combined) is 512 with any mix. The 512 inputs and 512 outputs are complementary, meaning that in order to use this much I/O, pairs of modules must have duplicate addresses. The maximum number of analog I/O is 512. 1785-L20B can utilize a total of 13 I/O chassis, and among them, 12 can be universal remote I/O chassis. The backplane current load of the 1785-L20B is 2.3 Amps at 5 Volts. 1785-L20B is not capable of connecting to an extended local I/O chassis.