Purchase now: Allen Bradley 1747-L20G Fixed Controller
Purchase now: Atlantechdrives provides you the 1747-L20G, a highly specialized, high-speed «Fixed Hardware» programmable logic controller from the Allen Bradley SLC 500 lineup. While standard fixed controllers handle 120V AC discrete signals, the 1747-L20G is entirely engineered for precise, rapid DC operations. It is equipped with 20 total I/O points: 12 inputs configured for 24V DC Sinking, and 8 outputs utilizing ultra-fast DC Transistor Sourcing architecture. What truly sets the L20G apart from the rest of the fixed family is its embedded High-Speed Counter (HSC). This makes the 1747-L20G the perfect, cost-effective standalone controller for rotary encoder tracking, high-speed packaging sorters, cut-to-length machines, and rapid bottling operations where standard PLC scan times are simply too slow.
Technical Specifications of the 1747-L20G
This controller integrates the processor, memory, and DC I/O into one compact footprint. The solid-state DC transistor outputs provide lightning-fast switching times (0.1 milliseconds), allowing for precise actuation of DC solenoids based on high-speed encoder feedback.
- Part Number: 1747-L20G
- Controller Type: Fixed Hardware Style
- Total I/O Points: 20 Points
- Inputs: Twelve (12) 24V DC Sinking (NPN logic)
- Outputs: Eight (8) 24V DC Transistor Sourcing (PNP logic)
- Special Feature: Embedded High-Speed Counter (HSC)
- Line Power Requirement: 120 / 240V AC
- Memory Capacity: 1K Instructions
- Communication: DH-485
Expert Tips: Sinking Inputs and High-Speed Counting
Integrating encoders requires an exact understanding of this controller’s hardware logic. Expert Tip: The 12 inputs on the 1747-L20G are «Sinking» inputs. This means you must use «Sourcing» (PNP) field devices—including your rotary encoders—to feed a +24V DC signal into the input terminals. The embedded High-Speed Counter bypasses the normal software scan cycle (which takes several milliseconds) and processes encoder pulses in real-time via dedicated hardware interrupts. If you attempt to read a fast-spinning encoder using standard inputs on an L20A, you will miss counts and cut your material to the wrong length. The L20G guarantees hardware-level precision.
Installation Advice: Transistor Protection
The 8 DC transistor outputs on this unit are incredibly fast but physically delicate compared to heavy mechanical relays. When using the 1747-L20G to drive inductive DC loads, such as small motor starter coils or pneumatic valves, the collapsing magnetic field of the coil will generate a reverse voltage spike (inductive kickback). If this spike hits the sourcing transistor, it will blow the silicon internally, permanently ruining that output channel. You must install a flyback diode (such as a 1N4004) across every inductive load to safely dissipate this energy and protect your controller investment.
Frequently Asked Questions (FAQs)
Q: Can I connect 120V AC sensors to the inputs of the 1747-L20G?
A: No. Applying 120V AC to the 24V DC sinking input terminals will cause an immediate short circuit and destroy the internal optocouplers. The field devices must operate on 24V DC.
Q: How do I program the High-Speed Counter?
A: The HSC is configured via the advanced setup files in RSLogix 500. You will map the physical input channels dedicated to the counter to trigger specific interrupt routines within your ladder logic.
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For standalone machine control, this fixed 20-point SLC unit provides reliable performance in a compact footprint.
