Field Wiring Best Practices for DI Modules

This guide covers best practices for wiring digital inputs on BAS controllers, specifically addressing shared vs individual return wiring.

The Question: Shared Return vs Individual Return

When wiring multiple dry contact inputs (door switches, status contacts, etc.) to a DI module:

Option A: Shared Return

Option B: Individual Returns

Best Practice: Individual Returns

Recommendation: Use individual return wires for each digital input.

Reasons for Individual Returns

Benefit	Explanation
Easier troubleshooting	Isolate problems to specific circuit
Better signal integrity	No shared return path noise
Matches field wiring	18/2 cables have dedicated pair
Proper isolation	Each device electrically independent
Simpler documentation	1:1 terminal mapping
Reduced crosstalk	Important for sensitive inputs

When Shared Returns Are Acceptable

Shared returns may be OK when:

• All inputs are on same equipment (e.g., multi-pole relay)
• Very short distances (within same enclosure)
• Common ground is already established
• Controller specifically designed for shared common

Wiring Standards

Typical Field Cable: 18/2 Shielded

For each digital input:

• Red wire: Signal (to input terminal)
• Black wire: Return (to common terminal)
• Shield: Ground at controller end only

Terminal Assignment Example (Individual Returns)

For SpaceLogic DI-16 or similar:

Field Device	Cable	Signal Terminal	Return Terminal
Door Contact	18/2 #1	DI-1	COM-1
Fan Status	18/2 #2	DI-2	COM-2
Filter DP	18/2 #3	DI-3	COM-3

Note: Some controllers have dedicated COM per input; others have grouped COMs. Check controller documentation.

Controller Considerations

Common Types of DI Modules

Type 1: Individual Commons

Best for individual return wiring (1:1 mapping).

Type 2: Grouped Commons

Acceptable, but individual cables still recommended for field runs.

Type 3: Single Common

Shared return required; individual field cables still land at single COM.

SpaceLogic DI-16 Specifics

The Schneider SpaceLogic DI-16 module:

• 16 digital inputs
• Grouped commons (typically 4 inputs per common)
• Still recommended: Individual return wires from field
• At controller: Multiple returns can share grouped COM

Installation Guidelines

Do's

• Do use 18/2 shielded cable for each input
• Do land each return on nearest available COM terminal
• Do label both ends of every wire
• Do maintain consistent color coding (signal = red, return = black)
• Do ground shields at controller end only

Don'ts

• Don't daisy-chain common connections in the field
• Don't use single wire with common return across multiple devices
• Don't mix power supply returns with signal returns
• Don't exceed voltage ratings (typically 24VDC)

Troubleshooting Advantages

With individual returns, troubleshooting is straightforward:

Problem: Single input not reading correctly

With Individual Return:

1. Disconnect cable at controller
2. Measure continuity/resistance of that cable only
3. Verify field device operation
4. Repair specific circuit

With Shared Return:

1. Multiple inputs may be affected
2. Must isolate which device is problematic
3. Shared path makes diagnosis harder
4. Potential for multiple fault points

Cost Considerations

Argument for shared returns: "Save wire and labor"

Reality check:

• Field devices typically come with 18/2 whips
• Running 18/2 vs adding shared return = minimal difference
• Troubleshooting costs far exceed wiring savings
• One difficult diagnosis = weeks of wiring "savings"

Documentation

Regardless of wiring method chosen:

1. As-built drawings showing terminal assignments
2. Wire schedules with cable numbers
3. Device schedules with point names
4. Color code legend for site

Summary

Approach	Recommendation
Individual returns	Preferred - Better isolation, easier troubleshooting
Shared returns	Acceptable only when controller requires it
Mixed approach	Avoid - Inconsistent and confusing