Duplex hotspot timeout issue resolved by firmware update

posted in: DMR, Ham radio | 4

A few weeks ago I posted about a few problems I encountered using the Duplex MMDVM hotspot, one of which being, what appeared to be a 60 second timeout.

Following this discovery I contacted Andy CA6JAU who writes the MMDVM_HS firmware, and he was able to reproduce similar problems on his duplex hotspots, but with a timeout which was consistently over 60 secs.


Andy fairly quickly tracked down the problem, and let me know that …


The root of the problem is clock bit rate drift between radio and HS. Because this depends on TCXO small differences (radio and HS), I modified a little the code to minimize this effect. At least here works for all my duplex boards.

Andy released version 1.4.7 to resolve this issue, but he has indicated that this is not a perfect solution.
It just increases the time before the transmitting rig (e.g. GD-77) will disconnect, because ultimately the reason for the timeout is that the TX chip (ADF7021) on the duplex hotspot, does not have its clock signal locked to the incoming signal (e.g. from my GD-77) and over a period of time, either the data buffer in the hotspot under-runs’ (empties) , or over-runs (overflows).

I know some people including Steve K2GOG are investigating using a clock clock rate generator chip, as the clock source for both the Rx and Tx ADF7021, as the clock frequency could be adjusted by the firmware. However I’m not sure if the cheap clock rate generators, like the Silicon Labs Si5351, have too much jitter to be be usable.
If I hear from Steve that he’s got this working, I have a couple of Si5351 breakout boards, so I could attempt to modify one of my duplex hotspots, and I will write a post about it.


In the mean time,… I tested 1.4.7 and my timeout is now 4 minutes.
I’m not sure if this timeout is being caused by the Brandmeister network, server or whether my 60 secs timeout has been multiplied by 4 to now be 4 minutes.

But I think this is an acceptable timeout as most repeaters enforce a 3 minute timeout. So to all intends and purses this bug has been fixed.



4 Responses

  1. Robert Fullarton

    Just found this site about txpreamble.


  2. Roger Clark


  3. Aristotle Zoulas

    Hello, I have MMDVM DUPLEX hotspot from N5BOC. Works well except for the timeout issue that it seems many of if not all the duplex boards are having. I have tried various firmware levels including the latest, After several minutes of receiving the MMDVM modem seems to disconnect. If I key up again, it will continue for a few more minutes, then the cycle continues. I have a $50 home made Chinese simplex JumboSpot that does not have this problem. It will stay connected to a talkgroup indefinitely (tested for one week straight). When I configure a static talk group on Brandmeister, the problem goes away. The problem seems to be independent of the radio. I have tried Ailunce HD1 and Motorola XPR6550. No difference at all .Is there any chance this will ever be resolved? Its real annoying.

    Thanks, Aristotle Zoulas W2ZOU

  4. Roger Clark

    From what I understand from Andy CA6JAU, who wrote the firmware….

    The timeout problem is caused by a difference in the “clock” frequency in the radio and the hotspot.

    The problem occurs because although the receiver chip (ADF7021) (its actually a transceiver chip), can recover the underlying clock signal from the incoming digital signal e.g. from your Ailunce HD1, the ADF7021 does not have an output pin for this critical signal, and slowly the signal from the transceiver gets out of sync with the internal clock on the duplex board.

    I’m not sure why this only happens on the duplex board, but its probably more to do with the way the transceiver e.g. Ailunce HD1 works, as I think in duplex, the transceiver listens for response data from the hotspot and its probably the response data that gets out of sync.

    According to Andy, there is no way to fix it in software. He already did a fix to introduce a buffer to try to keep the data in sync, but sooner or later the buffer either gets too full or gets emptied (depending on which clock frequency is higher, the hotspot or the transceiver).

    There may be a hardware fix for this, by using a clock generator IC like the Si5351 driven by the TCXO, so that the firmware could adjust the clock frequency of the Tx ADF7021, but as far as I am aware, no one has built a board with that feature in it yet (and I’m not even sure if the Si5351 is capable of generating accurate enough clocks for DMR, even if fed with the output from a TCXO as its reference)

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