The Allen-Bradley 1775-KA, also cataloged as the 1775 communication adaptor module, operates as a dedicated hardware component for bridging PLC-3 processors onto the Allen-Bradley Data Highway peer-polled network within legacy 1775 chassis systems.

Catalog Number Decoding

• 1775-: PLC-3 family chassis and interface module series
• KA: Data Highway (DH) communication adaptor, PLC-3 to DH link, standalone 115/230 VAC powered

Hardware Specifications

PLC-3 Backplane Coupling and Protocol Translation

The 1775-KA is not a rack-plugged PLC-5 module; it mounts in a 1775 PLC-3 chassis bay and draws its own 115/230 VAC from a separate feed, not from the 1775 backplane DC bus. It performs asynchronous framing conversion between the PLC-3 peripheral bus and the DH half-duplex 57 kbps tokenless peer-poll protocol. There is no firmware flash path—personality is mask-ROM fixed per revision tape label. I/O density scaling does not apply; one 1775-KA services exactly one PLC-3 processor node on the DH trunk. EtherNet/IP or Profinet licences are absent; this is pre-ControlLogix legacy serial networking. Backplane bus velocity licence is irrelevant—the module talks DH coax, not a backplane.

Ordering Info

• 1 x 1775-KA communication adaptor module (discontinued, surplus sealed unit)
• 1 x 1770-CD DH coax interconnect lead set (sold separately, not included)
• 2 x 75 ohm termination resistors (field-supplied per DH segment end rule)

Panel and Coax Installation Rules

• Bolt module into 1775 chassis slot with factory mounting ears; torque 2.3 N·m, do not cantilever without bottom bracket.
• Feed 115 VAC or 230 VAC (select via rear voltage selector) from isolated control transformer, 35 W minimum, separate from 24 VDC sensor supply.
• Run DH coax (1770-CD) with 75 ohm characteristic impedance; terminate both physical ends with 75 ohm resistors, no mid-span taps beyond 32 nodes.
• Keep DH coax 200 mm from 480 VAC power trays; cross at 90 deg only.
• Bond module chassis ground to PLC-3 cabinet ground bus with 4 mm² green/yellow, single point, no shield daisy-chain.

Frequently Asked Questions

Q: Can the 1775-KA be installed into a PLC-5 1785 chassis?

A: No. Physical card height and edge connector differ. PLC-5 uses 1785-KA or 1771-KF/KE for DH+. Forcing 1775-KA into 1785 backplane misaligns the 115 VAC power tab and bends the DH header pins.

Q: Is the module hot-swappable on a live PLC-3?

A: No. The 1775-KA carries line voltage on the rear connector. Removing under power arcs the 115/230 VAC tabs and corrupts the DH peer table; PLC-3 logs node loss F61. De-energize chassis first.

Q: What happens if 75 ohm terminators are omitted on DH coax?

A: Signal reflects at 57 kbps; peer-poll collisions rise, CRC errors climb, PLC-3 DH read/write times out at 750 ms. Both ends must be terminated, mid-trunk tee joints are not substitutes.

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

A: No. It speaks original DH (57 kbps, half-duplex). DH+ uses 1785-KA in PLC-5 or 1770-KF2 bridges. Mixing DH and DH+ coax on one 1775-KA port produces no link LED and silent timeout.

Q: Power draw when idle vs active?

A: 35 W max at 230 VAC full DH traffic. Idle (PLC-3 stopped, DH quiet) draws approx 18 W from the 115 VAC rail. Undersizing feed transformer below 50 VA causes brownout reset at power-on surge.

Q: Can firmware be updated in field?

A: No. Mask-ROM locked, no flash socket. Revision changes require factory board swap. Surplus units carry the tape label revision (typically Rev B/C); mixing revisions on one DH trunk is allowed but config file must match node table.

Q: Maximum DH trunk length with 1775-KA?

A: 7000 cable-feet (2133 m) using 1770-CD coax, 57 kbps, 75 ohm terminated, 32 nodes max. Exceeding length degrades poll margin; repeaters (1770-KF2) required beyond that, not another 1775-KA.

Q: Isolation rating between DH coax and AC mains?

A: 1500 VAC dielectric withstand for 1 minute per Allen-Bradley 1775 bulletin. DH shield is capacitively coupled to chassis ground; do not tie DH shield to both cabinet ground and remote end shield simultaneously (ground loop).