Description
Product Introduction
The Allen-Bradley 1771-OBD is a 16-point DC digital output module engineered for the 1771 Universal I/O platform used with PLC-5 systems and 1771-ASB remote I/O chassis. It provides sourcing-type solid-state outputs across an operational voltage range of 10 to 60VDC, making it suitable for driving 24VDC or 48VDC field loads.
Featuring optical isolation between the backplane logic and field circuitry, the 1771-OBD protects sensitive processor electronics from high-voltage field spikes. Its high continuous current capability (up to 2 A per channel) allows direct switching of heavy DC actuators, solenoid valves, contactor coils, and industrial indication lamps without requiring interposing relays.
Key Technical Specifications
| Parameter | Value |
| Outputs Per Module | 16 Non-Isolated Points (Sourcing / PNP Output Driver Type) |
| Operating Voltage Range | 10 to 60 VDC |
| Output Current Rating | 2.0 A max per point @ 60°C | 8.0 A total module surge limit |
| Minimum Load Current | 1.0 mA per point |
| Inrush Current | 4.0 A per point for 10 ms (repeatable every 1 second) |
| On-State Voltage Drop | 1.5 VDC max @ 2.0 A load |
| Off-State Leakage Current | 1.5 mA max @ 60 VDC |
| Backplane Current Draw | 350 mA @ 5 VDC |
| Optical Isolation | 1500 VAC for 1 minute (Field to Backplane) |
| Signal Delay | OFF-to-ON: 0.5 ms | ON-to-OFF: 1.0 ms |
| Field Wiring Arm | 1771-WH 20-Pin Wiring Arm |
Application Scenarios & The “Trench” Experience
The Trench Story
During a retrofit outage on a mining conveyor skid, a bank of 24VDC hydraulic solenoid coils stops energizing on a 1771-A4B remote I/O rack. Diagnostics show backplane logic commands are reaching the rack, but output status LEDs fail to turn on. Troubleshooting reveals that the field supply fuse feeding the V_{DC} terminal on the 1771-WH wiring arm was blown due to a shorted valve coil. Replacing the damaged solenoid and swapping in a tested 1771-OBD module restores control to the hydraulic actuators within minutes, preventing a complete site shutdown.
Specific Applications
- Heavy Solenoid Switching: Directly energizes high-current 24VDC and 48VDC directional pneumatic and hydraulic solenoids.
- Control Relay & Contactor Actuation: Drives interposing relay coils, motor starter coils, and main line contactors in power distribution panels.
- Stacklight & Annunciator Panel Driving: Controls high-draw DC indicator lamps, visual beacons, and audible alarms across process skids.
- DC Actuator Positioning: Switches power to linear DC actuators and motorized valve positioners in water treatment facilities.
- 1771-OBD
- 1771-OBD
Transparency SOP: Quality Assurance & Testing
Every 1771-OBD digital output module undergoes hardware load and signal integrity testing before joining active inventory:
- Card-Edge & Component Audit: We clean and inspect backplane gold card-edge fingers, inspect internal power MOSFET output drivers, and check for signs of thermal stress or blown traces.
- Backplane Logic Verification: The card is placed in a live 1771-A2B chassis paired with a PLC-5 controller to confirm proper 5VDC logic recognition and status LED activation.
- Full-Load DC Bank Burn-In: Every individual output point is connected to an active 24VDC load bank, sourcing a full 2.0 A load per point while verifying on-state voltage drop and off-state leakage.
- Thermal Stability & Overcurrent Test: The module runs under heavy load for two hours to ensure driver transistors operate within safe temperature parameters.
- Isolation & Fusing Check: Optical isolation circuits and onboard trace protection are audited to verify isolation barrier integrity between field wiring and the backplane.
The Veteran’s Tech Trap Guide
- ⚠️ Confirm Sourcing vs. Sinking (PNP vs. NPN): The 1771-OBD is a sourcing module (switches positive V_{DC} to the load). The load’s negative terminal must be tied directly to DC Common. If your field wiring requires a sinking module (switching DC Common), you must use the 1771-OBN module instead.
- ❗ Mind the 8.0 A Total Module Limit: Although individual points can support up to 2.0 A, the entire 16-point module cannot exceed 8.0 A total continuous current at any given time due to thermal limits on the backplane connector and wiring arm terminal.
- ⚠️ Use Flyback Diodes on Inductive Loads: When driving unsuppressed inductive field loads (solenoid coils, relay coils, magnetic brakes), always install reverse-biased suppression diodes across the coil terminals to prevent inductive back-EMF spikes from damaging the output transistors.
- ❗ Check V_{DC} Field Power Wire Gauge: Because 16 points sourcing up to 8 A pull significant DC power through the 1771-WH wiring arm, verify that supply feed conductors are properly sized (minimum 14–16 AWG) to prevent excessive voltage drop under simultaneous coil actuation.
Frequently Asked Questions
Q: What is the difference between the 1771-OBD and the 1771-OBN?
The 1771-OBD is a DC sourcing module (supplies +V_{DC} out to the load). The 1771-OBN is a DC sinking module (switches the ground/common side of the load).
Q: What field wiring arm is required for the 1771-OBD?
The 1771-OBD uses the 1771-WH 20-pin field wiring arm.
Q: Can the 1771-OBD operate at 12VDC and 48VDC field voltages?
Yes. The 1771-OBD supports a wide input voltage window of 10 to 60VDC, making it fully compatible with standard 12VDC, 24VDC, and 48VDC power supplies.
Q: Are the outputs individually fused on the 1771-OBD module?
No. Outputs are non-isolated and do not feature individual onboard channel fuses. Branch fusing or fuse terminal blocks should be installed in the panel wiring to protect individual field devices.
Q: Can I use the 1771-OBD in a ControlLogix system?
Yes, provided the 1771 chassis is connected via a 1771-ASB remote I/O adapter communicating back to a ControlLogix processor equipped with a 1756-RIO scanner card or 1771-to-1756 migration module.
Q: What warranty covers your surplus and refurbished 1771-OBD cards?
Every 1771-OBD module we ship includes a full 12-month replacement warranty. If a hardware fault or output driver failure occurs under normal operating conditions within 365 days of purchase, we replace the unit immediately from stock or issue a full refund.
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