Intermittent reads and ghost reads on Wiegand

Likely causes

• Run too long or conductors too small for the reader's Wiegand spec

  ~25%

  Same reader on a short lead at the panel reads perfectly every time; installed, it's flaky, and it gets worse at night as the supply sags. Wiegand is commonly quoted around 150 m / 500 ft, but the real limit is the reader OEM's and it moves with conductor size.

• Shield or ground problem: floating, bonded at both ends, or reader referenced to a different supply

  ~20%

  Spurious invalid-format and unknown-card events that track with motors, VFDs, elevator gear or lighting contactors switching. Scope Data0 and Data1 and you'll see pulses with no card present.

• No common 0 V between reader power and the panel input

  ~18%

  Wiegand data is ground-referenced. Measure potential between the reader's negative and the panel's data common: more than a fraction of a volt and you've found it.

• Bad termination: one strand in a push terminal, nicked conductor, crimp on a data line

  ~17%

  Badge repeatedly while flexing the cable. A read that changes state under flex localises the joint. Loop resistance can measure perfectly fine and still be intermittent under movement.

• Reader browns out at the moment of read

  ~10%

  Meter on min/max, or a scope, on the reader's Vdc shows a dip below its rated minimum at the exact instant of presentation: the LED, sounder and read all draw at once.

• Data pair run alongside power, or long unterminated tails

  ~6%

  Intermittence follows load switching in the building. A temporary lead on a different route clears it completely.

• Short read range mistaken for a data fault

  ~4%

  Card has to touch where it used to read at a distance, but the reads that do happen are clean. That is a range problem, not a data problem. Go to Read range short at one reader before you re-pull anything.

What to bring

• DMM with min/max
• Oscilloscope or Wiegand decoder
• Known-good card
• Known-good reader
• Clamp meter for ground loop current
• Cable tracer
• Ladder

Steps

1. 1

   Step 1: Separate a range problem from a data problem first

   Clean reads at short range, nothing at distance = range, mounting or RF; go to Read range short at one reader. Reads that fail or come through corrupted at any range, plus invalid events nobody caused = data path. They get confused constantly and the fixes are nothing alike.

   If that doesn’t do it

   Meter reader power under read.

2. 2

   Step 2: Measure reader Vdc at rest and during a read

   Min/max hold on the meter, or a scope if you have one. Any excursion below the reader's stated minimum is a power fault, full stop.

   If that doesn’t do it

   Go to grounding and shielding.

3. 3

   Step 3: Enforce one common 0 V and one shield bond

   Bond the shield at the head end only and leave it open at the reader. Confirm reader negative and panel data common are the same 0 V. Check for ground-to-ground potential and loop current.

   If that doesn’t do it

   Wiggle test the terminations.

4. 4

   Step 4: Wiggle test every joint from the reader back

   Badge on repeat while you flex at the pigtail, the mullion, the hinge transfer, the frame knockout and the panel terminal. Re-terminate only what the wiggle test condemns, nothing else.

   If that doesn’t do it

   Verify the actual run length.

5. 5

   Step 5: Trace the real run length and conductor size

   Measure or trace it: the installed run is almost always longer than the drawing. Past the OEM's Wiegand distance the fix is a converter, a closer controller, or OSDP.

   If that doesn’t do it

   Bench-prove reader and panel.

6. 6

   Step 6: Bench-prove at the panel on a short lead

   Same reader, same panel port, a couple of metres of cable. Flawless reads there prove reader and panel good and cable bad. Now you know where the money goes.

   If that doesn’t do it

   Price the OSDP conversion properly, or re-pull.

7. 7

   Step 7: Convert the link to OSDP, but price the prerequisites, not just the config

   RS-485 differential handles distance and noise Wiegand can't, and it's supervised. But this is a hardware decision: you need an OSDP-capable reader (a Wiegand-only reader has to be replaced), an OSDP-capable panel port or a converter at each end, a twisted pair for A and B, and a reference conductor. Typical installed Wiegand cable is untwisted multi-conductor. It will work over short runs and it is the wrong cable for the long marginal run you are trying to fix.

   If that doesn’t do it

   Re-pull the right cable, or relocate the controller closer to the door.

More Access Control guides

• Reader dead: no light, no beep
• Valid read, door never unlocks
• Denied at certain times or certain doors
• Card works at one door, not another
• Nuisance door held and forced alarms
• Read range short at one reader
• Keypad won't take the PIN

All 54 guides