The Allen-Bradley 1775-L4, also cataloged as the 1775-L main processor module, operates as a dedicated hardware component for executing ladder scan and I/O supervision across the PLC-3/10 peripheral bus within 1775-A3 chassis systems.

Catalog Number Decoding

• 1775-: PLC-3/10 chassis and processor family
• L4: Level-4 main CPU for PLC-3/10, 16-bit custom ASIC, 2K-word data table baseline

Hardware Specifications

PLC-3 Peripheral Bus Coupling and Firmware Lock

The 1775-L4 seats in the dedicated CPU bay of a 1775-A3 chassis and draws 5 VDC from the 1775-P3 power supply; it is not 1785-Lxx pin-compatible and will not seat in a 1771 PLC-5 rack. Execution closes over the PLC-3 peripheral bus at 10 ms/K instruction scan, with I/O mapped through a 1775-SR5 or 1775-S4 scanner module—not directly on the backplane. Firmware is mask-ROM fixed; no flash update path exists. Memory expansion uses 1775-MS4 (32K) / MSA (64K) daughter cards stacked on the CPU header. EtherNet/IP, ControlNet and Profinet licences are absent—networking is limited to DH 57.6 kbps twinax and 1.544 Mbps RIO. Backplane bus velocity licence does not apply; this is a 1980s 16-bit peripheral-bus CPU, not a ControlLogix rack module.

Ordering Info

• 1 x 1775-L4 main processor module (discontinued 2014-04-29, surplus sealed unit)
• 1 x 1775-MS4 32K memory daughter card (optional, not fitted)
• 1 x 1775-CAK front-panel interconnect cable (field-supplied)

Chassis Mounting and Bus Rules

• Seat CPU in the leftmost designated processor bay of 1775-A3 chassis; do not occupy I/O scanner slot with the L4.
• Feed 1775-P3 power supply from isolated 115/230 VAC, 5 VDC backplane rail must settle at 5.0 VDC +/-0.25 V before CPU insert.
• Terminate DH twinax (Belden 8723) with 75 ohm resistors at both trunk ends; max 32 nodes, max 7000 ft trunk.
• RS-232 programming port uses 25-pin D-sub to 1770-T4 terminal or PC null-modem; DF1 protocol at 9600/19200 bps.
• Bond 1775-A3 chassis ground to cabinet ground bus with 4 mm² green/yellow, single point only.

Frequently Asked Questions

Q: Can the 1775-L4 be inserted into a PLC-5 1785 rack?

A: No. Edge connector keying and backplane signaling differ. 1775-L4 runs only in 1775-A3 (PLC-3/10) chassis. Forcing into 1771-A3B bends the 5 VDC tabs and leaves the 1785 bus undriven.

Q: Is the module hot-swappable on a live 1775 backplane?

A: No. 5 VDC backplane insertion arcs the ASIC power pins and corrupts the peripheral bus arbiter. PLC-3/10 requires 1775-P3 power off before CPU extraction or insertion.

Q: How is user memory expanded beyond 2K words?

A: Stack 1775-MS4 (32K), 1775-MS8 (32K) or 1775-MSA (64K) CMOS RAM daughter cards on the CPU header. Data table then spans base 2K plus daughter card space; no firmware change needed.

Q: What scan time applies to 4K boolean instructions?

A: 10 ms per 1K instructions nominal, so 4K executes in 40 ms per full scan. Timer/counter and PID blocks add overhead; actual wall-clock scan is 10-15% longer than pure boolean math.

Q: Does 1775-L4 support DH+ (Data Highway Plus)?

A: No. It speaks original DH at 57.6 kbps half-duplex peer-poll over twinax. DH+ (1784-KT/XT) is a PLC-5 era protocol; bridging requires a 1775-KA to DH then a 1784-KF2 gateway.

Q: Can RS-232 port run while DH trunk is active?

A: Yes. RS-232 is independent DF1 point-to-point for programming/HMI; DH trunk runs peer-poll concurrently. Both share the 16-bit ASIC but separate UART blocks; no arbitration conflict.

Q: What replaces 1775-L4 in a migration?

A: No drop-in. PLC-3/10 migrates to ControlLogix 1756-L7x with 1783-ETAP DH+ tap and 1756-DH/DHRIO bridge modules. Ladder translates via RSLogix 5->5000 conversion, not binary lift.

Q: Battery backup on 1775-L4?

A: No on-board battery. CMOS RAM (1775-MS4/MSA) holds data only while 1775-P3 5 VDC rail is alive; on total power loss data table clears. Program resides in mask-ROM; reload via 6200 software from tape/PC.