A Comprehensive Guide to the 1747-CP3 Cable
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In the world of industrial automation, shiny new Ethernet-enabled controllers often get the spotlight. However, the true "unsung heroes" of the factory floor are often the legacy systems that have been running reliably for decades. Among these, the Allen-Bradley SLC 500 series remains a staple of American and international manufacturing. If you have ever had to maintain, program, or troubleshoot an SLC 5/03, 5/04, or 5/05, you know that your connection is only as good as your cable.
Enter the 1747-CP3.
This specific cable is the standard serial communication link required to connect a personal computer to the RS-232 port of an SLC 500 processor. While it may look like a simple "wire," the 1747-CP3 is a precision-engineered tool that follows a specific null-modem wiring scheme essential for industrial handshaking. In this guide, we will explore why this cable is indispensable, how it is wired, and how to configure it in the modern era of USB-only laptops.
Defining the 1747-CP3
The 1747-CP3 is a 9-pin D-shell (DB9) female to 9-pin D-shell female serial cable. It is specifically designed for point-to-point communication between a computer's COM port and the Channel 0 (RS-232) port on an SLC 500 controller.
Supported Controllers
While many cables in the Allen-Bradley ecosystem are proprietary, the 1747-CP3 is most commonly used with the following hardware:
- SLC 5/03: Connects via the bottom DB9 port.
- SLC 5/04: Connects via the bottom DB9 port.
- SLC 5/05: Used for initial IP address configuration or backup communication via the serial port.
- MicroLogix Controllers: While MicroLogix typically uses the 1761-CBL-PM02 (mini-DIN), the 1747-CP3 can be used with some MicroLogix 1500 bases and specific port adapters.
Internal Anatomy: Null-Modem Wiring
The most common mistake maintenance technicians make is assuming a standard, "straight-through" serial cable from a local electronics store will work. It will not.
The 1747-CP3 is a Null-Modem cable. In a straight-through cable, Pin 2 connects to Pin 2. In a null-modem configuration, the Transmit (TX) and Receive (RX) lines are crossed. This allows two "Data Terminal Equipment" (DTE) devices—the PC and the PLC—to talk to each other directly.
PC Side (DB9 Female)
SLC Side (DB9 Female)
Function
Pin 2 (RX)
Pin 3 (TX)
Receive to Transmit
Pin 3 (TX)
Pin 2 (RX)
Transmit to Receive
Pin 5 (GND)
Pin 5 (GND)
Signal Ground
Pin 7 (RTS)
Pin 8 (CTS)
Request to Send to Clear to Send
Pin 8 (CTS)
Pin 7 (RTS)
Clear to Send to Request to Send
Pin 4 (DTR)
Pin 6 & 1 (DSR/DCD)
Data Terminal Ready to Data Set Ready
Configuration in RSLinx
Once the physical 1747-CP3 cable is connected, the next hurdle is software configuration. Most users struggle here because RS-232 is not "Plug-and-Play" like USB. You must tell the software exactly how to talk.
Step-by-Step Setup:
- Open RSLinx Classic.
- Navigate to Communications > Configure Drivers.
- Select RS-232 DF1 Devices from the dropdown and click "Add New."
- Select the correct COM Port (Check Device Manager if using an adapter).
- Select Device Type: SLC-CH0/Micro/PanelView.
- Click the "Auto-Configure" button.
If your cable is wired correctly and the port is active, RSLinx will cycle through baud rates and parities until it shouts "Auto-Configuration Successful!" This moment is the gold standard for any PLC technician—it confirms that your 1747-CP3 is doing its job.
USB to Serial Adapters
In 2026, finding a laptop with a native 9-pin serial port is nearly impossible. This means your 1747-CP3 needs a "partner": a USB-to-Serial Adapter.
However, not all adapters are created equal. Industrial communication requires high-speed timing and specific voltage levels. Cheap adapters often fail to "Auto-Configure" because their chipsets (like low-end Prolific clones) cannot handle the rapid toggle of the handshake lines.
Pro-Tips for Success:
Look for FTDI Chipsets: Always use a USB-to-Serial adapter with a genuine FTDI chipset. They are the industry standard for stability with Allen-Bradley software.
COM Port Numbering: Windows often assigns USB adapters a high COM port number (like COM12). RSLinx sometimes struggles with ports higher than COM8. You can manually reassign the port to COM1 or COM2 in the Windows Device Manager under "Port Settings > Advanced."
Industrial Durability and Environment
Why buy an official 1747-CP3 instead of a generic null-modem cable? It comes down to the environment.
- Shielding: Industrial cables are shielded to prevent Electromagnetic Interference (EMI) from nearby Variable Frequency Drives (VFDs) and high-voltage motors.
- Strain Relief: The molded boots on a 1747-CP3 are designed to withstand being pulled through tight cabinet gaps or being stepped on by steel-toed boots.
- Thumb Screws: Standard serial cables often have short screws. Industrial versions typically feature oversized, easy-to-grip thumb screws for quick connection without a screwdriver.
Common Failures
If you have the cable, the adapter, and the software, but you still see "Comm Error," check these three things:
Port Conflict: Is another program (like a mouse driver or another PLC software) trying to use the same COM port?
Default Settings: If "Auto-Configure" fails, the SLC 500 port might have been changed from the default DF1 protocol to DH-485 or ASCII. If it is set to DH-485, the 1747-CP3 will not work directly; you would need a 1747-UIC converter instead.
The "Keyhole" Reset: If the port is totally unresponsive, you may need to perform a factory reset on the processor (removing the battery and shorting the VBB/GND pads) to return Channel 0 to its default DF1 settings.
The 1747-CP3 is more than just a cable; it is a bridge to the past that keeps the present running. Whether you are performing a routine backup of a 20-year-old SLC 5/04 or troubleshooting a critical machine at 2:00 AM, having a reliable, correctly wired 1747-CP3 in your toolbag is non-negotiable.
While the world moves toward wireless and Ethernet, the physical, tactile reliability of a serial connection ensures that as long as there is power to the processor, there is a way to see what’s inside.
Is your toolbag ready? Ensure you have a high-quality 1747-CP3 and a verified FTDI adapter before your next service call. It’s the difference between a 5-minute fix and a long night of frustration.
