Hx1 or Hx2 freezes or locks up, requiring reboot


When a call is placed on hybrid, it's possible for the Hx1 or Hx2 to lock up and not respond to user input until power is cycled. Here, we'll explain why this happens.

Loop Voltage

Every standard POTS line has 2 wires (tip and ring) with a DC voltage across it. The Hx systems rely on signaling, in the form of changes to this DC voltage, to operate correctly. When the line is "On Hook" the voltage across tip and ring will be the full 48-50 V DC. When a telephone on this line goes "Off Hook," a load (the telephone) is placed across this voltage, which pulls it down to a much lower value. This tells everything on the circuit, including the device providing dial tone, that the line is in use.

When the Hx is switched on, it stops and waits for full loop voltage to appear before fully switching the line to the hybrid. The Hx will stay in this state until it either finds loop voltage or until it's powered down.

So if you turn on the hybrid and it stops working or responding to you, it's because it's waiting for loop voltage and can't find it.

Why can't the Hx find loop voltage?

  1. The phone line is in use elsewhere.

The Hx is designed to avoid accidents on the air, such as airing a call before the screener (using the connected POTS phone, or an extension of the line) is done talking to the caller. It is waiting for the telephone to be placed back on hook (loop goes back up to full voltage) before switching the line to the hybrid.

  1. Connections to the LINE and PHONE jacks are reversed

In idle state (Hybrid off), the LINE and PHONE jacks are connected together to facilitate use of the POTS phone connected to the PHONE jack. If the connections are reversed, the POTS phone will work fine while the Hybrid is switched off.

When the hybrid is switched on, the PHONE jack is disconnected and the LINE jack is connected to the hybrid. With these connections reversed, the loop voltage (connected to the PHONE jack) goes nowhere as the Hx looks for it on the LINE jack, where it won't find it at this point. It remains in this state until reboot, or until the connections are reversed and the loop voltage appears at the LINE jack.

  1. Loop voltage has a current limit that is too low

The Hx expects loop voltage to drop to a low value once it connects the phone line to the hybrid, but it does not expect the voltage to drop all the way to 0V. If the current limit on the line is too low, the voltage will drop too far when a load is placed across it. We expect about 30-60mA to be available at the POTS FXS interface.

Sometimes ATAs (Analog Telephone Adapters) that simulate phone circuits can not generate loop with enough available current to drive a business class device like the Hx system. When all else fails, you can try a different ATA to see if the results change.

Still no good?

With a DC Voltmeter, measure the voltage across the POTS line (center pair of the RJ-11). If there's no voltage, that's the problem. You'll want to contact the provider and tell them there's no loop or trace the cable back to its origin and make sure everything is good there.

If you measure +/- 48-50 V DC across the phone line and you're certain that this cable is good and is plugged into the LINE jack of the Hx system, it may have been exposed to a surge event and will have to come into a repair center for inspection. In this case, contact us to start the RMA process.

Let us know how we can help

If you have further questions on this topic or have ideas about how we can improve this document, please contact us.

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