Allen-Bradley 1747-SN Remote I/O Scanner Module

The Allen-Bradley 1747-SN is a Remote I/O (RIO) Scanner Module for the SLC 500 PLC platform. Its primary purpose is to allow an SLC 500 CPU to communicate with and control I/O located at a distance (up to 10,000 feet) via a single “Blue Hose” (twinaxial) cable.

It acts as a gateway between the SLC backplane and a network of remote “Adapters” (like the 1747-ASB or 1794-ASB).

1. Key Technical Specifications

The 1747-SN is a single-slot module that occupies one slot in the SLC 500 chassis.

2. Wiring and the “Blue Hose”

The physical connection is made using the classic 1770-CD (Blue Hose) cable. Wiring is polarity-sensitive.

• Terminal 1: Line 1 (Blue wire)

• Terminal 2: Shield (Clear/Bare wire)

• Terminal 3: Line 2 (Clear/White wire)

• Terminating Resistors: You must install a terminating resistor at each physical end of the RIO link.

  • Use a 82 $\Omega$ resistor if the baud rate is 230.4k.

  • Use a 150 $\Omega$ resistor for 57.6k or 115.2k baud.

3. I/O Addressing and Data Mapping

The 1747-SN uses a method called Discrete Data Transfer to move data between the remote rack and the PLC memory.

• Scanned Image: The data from the remote I/O is mapped directly into the SLC input and output image tables ($I:e$ and $O:e$, where $e$ is the slot number of the 1747-SN).

• Logical Racks: The scanner views the remote network as “Logical Racks.” A single physical 13-slot rack might be configured as two “Logical Racks” depending on the dip switch settings of the adapter.

• Block Transfer (BTR/BTW): For complex modules like analog cards (NI4, NO4) or high-speed counters located in a remote rack, you must use Block Transfer instructions to move large chunks of data, as standard discrete scanning only handles simple On/Off bits.

4. Diagnostic LEDs

The front of the 1747-SN features LEDs that are essential for troubleshooting:

• RUN (Green): The scanner is operating correctly and scanning the network.

• FAULT (Red): Internal module failure or configuration error.

• COMM (Green/Flash): Indicates active communication on the RIO link. If this is off or solid red, there is a break in the cable or a baud rate mismatch.