PTI MUX Interface Board


PTI EOL boardAvailable from Access Security Ltd. this is a 16 channel monitored-line interface adaptor board for security sensor interfacing.

As is standard security system practice, end of line (EOL) resistors at the detector or sensor are switched to represent, closed open and tamper states. Faults such as short or open circuited wiring is detected as a tamper state.

Note: referenced below, Gnd is ground, -ve, common, etc. (depends on your terminology).


  1. There are 16 inputs
  2. Each require a 2.2K EOL resistor to Gnd to close (normal)
  3. 2.2K + 6.8K is the open (activated) state
  4. Short or open circuit is the tamper state
  5. 2.2K in series and 6.8K across the alarm contact (parallel)

This is standard security EOL wiring for single zone per input.

LED Indications

  1. There are 4 LEDs (red or green). One LED for each group of 4 inputs
  2. A slow strobe = all 4 input are closed (normal)
  3. Flashing = one or more inputs is open or in tamper

Strobe is a very brief flash. There will be a dim glow between flashes, this is normal.


  1. There are 16 zone outputs and 1 tamper output
  2. All are Open-Collector transistor outputs (sink outputs)
  3. Zone outputs are closed when the input is closed
  4. Zone outputs are open when the input is open or in tamper
  5. The tamper output is normally open, closing when any one or more input is in tamper

Open collector transistor outputs switch to Gnd; similar to a relay contact. There will always be a volt drop across the transistor; it does not switch hard to Gnd. Each output can sink 50mA.

Power Supply

The board requires 12V DC (9-14V DC safe range) at 80mA. This should be a clean regulated power supply from the security or PLC controller; not a unregulated plug-pack.

  1. +12V is positive. -ve or common is Gnd (just depends on your terminology)


PTI EOL boardNote: Installation and testing must be carried out by a qualified or suitably experienced security technician.

Have a set of input plugs with 2.2K EOL resistors fitted to test the installation before attaching any input wiring. This will confirm that the board is correctly connected and working before adding external variables.

The LEDs will strobe (brief flash) indicating all zones are closed. Shorting a resistor with a wire loop will cause the associated LED to flash and your system should register a zone activation and tamper; assuming your system is configured to do so.

  1. If the LEDs are strobing then the associated inputs are closed with the correct EOL resistor value
  2. A flashing LED indicates an input open or in tamper
  3. Ensure that the 2 EOL resistors are correctly wired (not swapped)
  4. Measure the voltage on the input referenced to Gnd

Input testing – Voltage readings

The only reliable input test is to measure the voltage at the input terminals. You cannot measure resistance while the input is connected to the zone wiring and detector, even when the board is powered off. If you must measure the EOL resistance you must unplug or disconnect the zone wiring from the board.

Open circuit Closed – Normal Activated – in alarm Short circuit
5V  ??V ??V 0V

The important states (voltages) are Closed and Activated. These voltage readings are +/- 100mV (0.1V).

Why measure voltage and not resistance?

Considering one zone: The EOL resistor/s form a voltage divider with a resistor on the board. The board measures the voltage produced and determines the state of the zone. By measuring the voltage you are seeing what the board sees without influencing the operation. It is an accurate representation of what is going on.

Measuring resistance with the board and zone connected, you are seeing a complex combination of the EOL resistors, and on-board resistors. If power is applied, your multimeter will be really confused, but not likely damaged.

Production and Production Testing

Production is SMD using a solder paste screen, component placement and reflow soldering. Edge connectors are added manually. Wire leads are added manually; tricky and time consuming. Wire cores are from solid core Cat5 (for IDE connection). Wire preparation involves, cutting to length, selection, straightening and stripping. Soldering the wires is a 2-step process; initial attachment, then adjusting to compensate for plastic shrinkage.

Visual checks:

  1. IC alignment and pin shorts
  2. LED alignment
  3. Continuity check for shorts on small input capacitors
  4. Alignment/short/open check resistor packs


The test rig consists of a 12V DC supply, array of 16+1 LEDs with resistors to +V. Two sets of four input connectors are used one fitted with 6K8 and one with 10K resistors. 6K8 represents closed, 10K represents open.

  1. Connect LEDs and apply power. Only the tamper/fault LED should be on, on-board LEDs flashing.
  2. Connect 6K8 plugs. Output lEDs on, tamper/fault lED off.
  3. GND each input in turn. Output LED goes off, tamper/fault LED comes on.
  4. Swap to 10K plugs. Output LEDs off, tamper/fault LED off, on-board LEDs strobe/pulsing.
  5. GND each input in turn. Output pulses on before tamper/fault comes on.