Allen-Bradley 1747-CP3 PLC Programming Cable – SLC 500 Series

Allen-Bradley 1747-CP3: RS-232 to DH-485 Direct-Connect Programming Cable for SLC 500 and MicroLogix Control Platforms

The Allen-Bradley 1747-CP3 is a factory-original, passive-active hybrid programming cable engineered by Rockwell Automation to establish a deterministic, point-to-point communication link between a PC RS-232 serial port and the DH-485 network port resident on SLC 5/03, SLC 5/04, SLC 5/05, and the full MicroLogix processor family. Unlike software-emulated adapters, the 1747-CP3 integrates a discrete RS-232-to-DH-485 level-shift circuit within the cable body itself, eliminating the need for an external communication module, a powered interface converter, or any additional hardware between the programming workstation and the target processor.

In a typical SLC 500 control architecture, the DH-485 port on the processor front panel serves as the primary programming and peer-to-peer messaging interface operating at differential signal levels compliant with EIA-485. The RS-232 port on a standard laptop or desktop PC operates at ±12 V single-ended UART logic. The 1747-CP3 bridges these two incompatible electrical domains through an embedded level-shift IC powered entirely from the RS-232 handshake lines (RTS and DTR), drawing no current from the PLC backplane or any external supply. This self-powered topology means the cable is operational the moment it is plugged in — no driver installation beyond RSLinx Classic is required, and no COM port power settings need adjustment on the host PC.

RSLinx Classic auto-negotiates baud rate with the SLC processor across the supported range of 1,200 bps to 19,200 bps. At 19,200 bps — the maximum DH-485 rate — the round-trip latency for a single RSLogix 500 online session packet is typically under 5 ms on a point-to-point link, which is sufficient for real-time data table monitoring, forced I/O operations, and program upload/download without perceptible delay. For multi-node DH-485 networks where the 1747-CP3 is used as a network node rather than a direct processor connection, the cable participates in the token-passing arbitration protocol native to DH-485, supporting up to 32 nodes on a single segment.

The cable assembly uses molded strain-relief boots at both connector ends and shielded, twisted-pair conductors for the DH-485 differential pair (A/B lines), providing measurable attenuation of common-mode noise in environments with variable-frequency drives, contactors, and high-current switching loads — the standard electromagnetic background of a discrete manufacturing or process control panel. The DB-9 male connector mates with the RS-232 port on the PC; the Mini-DIN 8-pin connector mates with the DH-485 port on the SLC processor or MicroLogix unit. Cable length is approximately 2 m (6.5 ft), which covers the typical distance between a laptop placed on a panel door and the processor mounted in the upper rack.

For plant maintenance engineers sustaining installed SLC 500 fleets, the 1747-CP3 remains the lowest-cost, lowest-complexity path to go online with a processor — no Ethernet infrastructure, no ControlNet bridge, no 1747-UIC USB adapter required when a DB-9 RS-232 port is available on the workstation. For OEM machine builders and system integrators performing factory acceptance testing (FAT) or site acceptance testing (SAT), the cable provides a self-contained, portable programming interface that requires no configuration beyond selecting the correct RSLinx driver.

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Technical Parameters

Hardware Logical Analysis

The 1747-CP3 internal circuit topology centers on a single-chip RS-232 to RS-485 transceiver — typically a MAX232-class or equivalent device — that performs bidirectional voltage translation between the ±12 V single-ended RS-232 domain and the ±5 V differential EIA-485 domain. The charge-pump capacitors required by the transceiver IC are integrated within the cable body, keeping the external footprint minimal.

Direction control on the half-duplex DH-485 bus is managed by monitoring the RS-232 RTS (Request to Send) line. When the host PC asserts RTS high prior to transmitting a packet, the transceiver switches the RS-485 driver to transmit mode; when RTS is de-asserted, the driver tri-states and the receiver is enabled, allowing the PLC response to propagate back to the PC RX line. This hardware-controlled direction switching eliminates the software-timed delays that plague some USB-to-RS-485 adapters, where driver latency can cause the transmitter to remain enabled long enough to corrupt the leading bytes of the PLC response.

EMC design: The shielded cable construction attenuates capacitively coupled noise from adjacent power conductors. The shield is terminated at the DB-9 connector shell, providing a low-impedance path to the PC chassis ground. On the DH-485 side, the differential signaling architecture provides inherent common-mode rejection — a 1 V common-mode noise signal on both A and B lines produces zero differential voltage at the receiver input, making the link immune to ground potential differences between the PC and the PLC panel of up to ±7 V (within EIA-485 common-mode range).

Token-passing arbitration: When the 1747-CP3 is used to connect a PC to a multi-node DH-485 network (rather than directly to a single processor), RSLinx Classic participates in the DH-485 token-passing protocol as a network node. The protocol assigns each node a station address (0–31) and passes a logical token sequentially. A node may transmit only when it holds the token, preventing bus collisions without requiring a dedicated network master. This deterministic arbitration ensures that programming traffic from the PC does not disrupt peer-to-peer messaging between SLC processors on the same segment.

System Integration Benefits

• Zero-hardware-overhead programming path: The 1747-CP3 requires no additional communication module (e.g., 1747-KE, 1747-KFC15) to establish an RSLogix 500 online session, reducing BOM cost and panel space consumption for commissioning and maintenance tasks.
• Deterministic round-trip latency: Point-to-point DH-485 at 19,200 bps delivers sub-5 ms packet round-trip time, enabling real-time data table monitoring with scan-cycle-level resolution — adequate for diagnosing timing-sensitive ladder logic faults.
• Self-powered operation eliminates supply dependency: Drawing logic power from RS-232 RTS/DTR lines, the cable operates independently of the PLC backplane power budget, ensuring the programming interface remains functional even during partial power-up sequences or backplane fault conditions.
• Hardware RTS direction control eliminates software timing errors: The embedded transceiver uses hardware RTS assertion for RS-485 bus direction switching, avoiding the transmitter-hold-time errors that cause corrupted response packets with software-timed USB adapters — a critical reliability factor during firmware download operations.
• Multi-node DH-485 network participation: The cable supports connection to a live DH-485 segment with up to 32 nodes, allowing the programming workstation to address any processor on the network without rewiring — useful for multi-axis or multi-zone SLC 500 architectures.
• Full RSLogix 500 feature access: Online programming, offline upload/download, forced I/O, data table monitoring, trend logging, and fault diagnostics are all accessible through the 1747-CP3 link — no feature restrictions compared to Ethernet-based communication paths for SLC 500 targets.
• Broad MicroLogix family coverage: A single cable model covers MicroLogix 1000, 1100, 1200, 1400, and 1500 via the common DH-485 port, eliminating the need to stock multiple cable variants for mixed-platform maintenance kits.
• Portable field commissioning tool: The ~350 g cable with 2 m reach is self-contained and requires no external power adapter, making it the standard tool for field service engineers performing startup, troubleshooting, and firmware updates at customer sites without access to a network infrastructure.

Quality Assurance & Global Logistics

Every Allen-Bradley 1747-CP3 unit stocked at siemensplc.com is sourced through verified supply channels and carries the original Rockwell Automation catalog label with date code for full authenticity traceability. Prior to dispatch, each cable undergoes a continuity check across all eight Mini-DIN conductors and a signal-integrity verification on the RS-232 DB-9 pins to confirm the internal level-shift circuit is functional. Units that do not pass electrical verification are quarantined and not listed for sale.

Shipments originate from our warehouse in Xiamen, China — a major logistics hub with direct access to DHL Express, FedEx International Priority, and UPS Worldwide Expedited services. Standard transit times to major industrial markets are 3–5 business days to Southeast Asia, 4–6 business days to Europe, and 5–7 business days to North America. All shipments include a commercial invoice, packing list, and — upon request — a certificate of conformance (CoC) and test report for OEM and system integrator procurement compliance requirements. Export documentation for cross-border customs clearance is prepared in accordance with HS Code 8544.42 (insulated electric conductors for a voltage not exceeding 1,000 V, fitted with connectors).

A 12-month warranty covers manufacturing defects in the cable assembly, connectors, and internal level-shift circuit from the date of shipment. Warranty claims are processed with a replacement-first policy — a replacement unit is dispatched upon receipt of the defective item and photographic evidence of the fault condition.

Contact Information

Email: [email protected]
WhatsApp: +86 18359268345
Web: siemensplc.com
Location: Xiamen, China
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