Technical Specifications
|
Parameter
|
Value
|
|---|---|
|
Product Model
|
1756-L1
|
|
Manufacturer
|
Allen-Bradley / Rockwell Automation
|
|
Product Type
|
ControlLogix 5550 (Logix5550) Processor / CPU Module
|
|
User Memory
|
64 KB (SRAM, battery-backed)
|
|
I/O Capacity
|
8 local I/O modules maximum, 128 maximum I/O connections
|
|
Backplane Bandwidth
|
128 kB/s sustained
|
|
Task / Program Model
|
16 tasks, 32 programs; periodic, event, and fault routines
|
|
Communication Ports
|
1 × RS-232 DB-9 (DF1 full/half-duplex, DF1 radio modem, DH-485); backplane to comm modules
|
|
Real-Time Clock
|
Battery-backed, used for time-stamped alarms and data logs
|
|
Memory Retention
|
Lithium battery, replacement 1756-BA1 (or 1756-BA2 on later Series)
|
|
Scan / Execution
|
0.08 ms/K typical Boolean; task-periodic scheduling, not fixed scan
|
|
Chassis Requirement
|
Any 1756 chassis, 1 slot, left-most or next to power supply per rack convention
|
|
Operating Temperature
|
0 to 60 °C, 5–95% RH non-condensing
|
|
Enclosure / Mount
|
Open-style, chassis/DIN-mounted, IP20 touch-safe
|
|
Dimensions (H×W×D)
|
142 × 102 × 82 mm (5.59 × 4.02 × 3.22 in.)
|
|
Approvals
|
UL 508, CSA C22.2, CE, C-Tick, FM Class I Div 2
|
|
Programming Software
|
RSLogix 5000 v2.01 through v10.x (project target: Logix5550, rev 1.xx–3.xx)
|
|
Lifecycle Status
|
Discontinued / obsolete – surplus, refurbished, or repair stock
|
Values are the published Bulletin 1756 ratings for the original 1756-L1 64 KB Logix5550. Series letter and firmware revision (typically 1.xx through 3.xx on the L1) must be read from the module label; later firmware cannot be loaded into an early-Series 1756-L1, and RSLogix 5000 v11 and above will not target the processor.
Main Features and Advantages
The architecture that invented the modern ControlLogix model
The Allen-Bradley 1756-L1 introduced producer/consumer communication, tag-based addressing, and task-driven execution to the Allen-Bradley world. Even at 64 KB, the controller does not think in I/O file numbers — it thinks in tags and structures. That means the machine program travels as named data, not as hard-coded I:3/7 references, which is why a 1756-L1 project is portable to a 1756-L55 or 5580 later with far less pain than a PLC-5 migration. The small memory is a constraint, but the data model is already current.
Eight modules is enough when the cell is one machine
The eight-local-module limit sounds restrictive next to a modern rack, but it was calibrated to the workcell the 1756-L1 was designed for: a 16-point isolated input module, an 8-point output, an analog input, a quadrature counter, and two or three communication or specialty modules. That is a complete machine. Plants that stay inside the envelope get deterministic behavior and a backplane that is never saturated, while the 128 kB/s bandwidth keeps the I/O updates invisible to the scan. For a standalone skid, the Allen-Bradley 1756-L1 is not underpowered — it is correctly sized.
Four LEDs and a key switch are all the diagnostics you need at 2 a.m.
The OK/FAULT/RUN/I/O legend on the 1756-L1 is deliberately primitive and that is its strength. A flashing OK means the processor has not passed its self-test; a solid FAULT with a mode-switch in Remote Run means the application faulted rather than the hardware dying; I/O lit means the backplane lost a module. Those four bits of information, read through a dirty panel window, tell the on-call tech whether to reach for a spare processor, reseat a module, or open RSLogix. There is no web page to time out and no Java console to launch.
The RS-232 port is a genuine service lifeline
Unlike later controllers that buried the programming path behind Ethernet, the 1756-L1 puts a real DB-9 on the front. DF1 point-to-point to a laptop, DH-485 to a PanelView 550, or a direct connection to an old DH+ bridge module — the port is there and it works when the network is down. For a legacy cell whose only documentation is a yellowed wiring diagram and a .RSS file on a retired engineer’s hard drive, that serial path is the difference between recovery and a forced rewrite.
Battery-backed certainty
The 64 KB SRAM and the real-time clock share the same lithium cell, so a power outage does not just preserve the program — it preserves the event sequence. When the cell dies, the Allen-Bradley 1756-L1 reports it clearly at boot and the program is recoverable from the archived .ACD (or the RSLogix 5000 export) before the next cycle. Fitting a fresh 1756-BA1 / 1756-BA2 is a five-minute panel-top job, and the discipline of keeping a spare battery behind the door is cheaper than explaining a lost shift log to quality.
A reference standard for migrations
Because the 1756-L1 is the smallest, cleanest expression of the Logix model, it is the ideal behavior baseline during a conversion. Run the existing logic on a tested Allen-Bradley 1756-L1, capture the tag table and task timings, and the replacement 1756-L55 or 5580 project has a known-good oracle to compare against. That reference value is why the processor still changes hands at real money despite being out of catalog for years.
