Duplex hotspot – MultiMode limitation

posted in: DMR, Ham radio | 0

I thought it was worth sharing a limitation with the MMDVM DUAL HAT duplex hotspot , which I uncovered today.


I was testing with both DMR and YSF mode enabled, and found that the GD-77 would only connect to the hotspot on a very intermittent basis.

Perhaps 1 in 10 tries would result in the GD-77 connecting.


My initial thought was that the hotspot may not be listening for DMR signals for long enough to detect the call setup data being sent by the GD-77, so I discussed this with Andy CA6JAU via a github issue, and he advised me that its a known “issue” and that I could try increasing the “SCAN_TIME” (hold time) setting in the firmware to see if that improved things, but that it would result in other modes being less responsive.


However before I attempted to change the firmware, and rebuilt it, used another FM receiver to listen to what the GD-77 was doing, and unfortunately it only appears to send one call setup burst of data, and does not appear to retry.

I did the same test with my MD-380 and it seems to try 3 times before giving up.


I’ve looked in the codeplug settings for the GD-77 and nothing I did seemed to make it try more than once to setup the call, and adjusting the preamble time didn’t make any difference at all to the bust of data that the GD-77 sends when it attempts to connect.


The overall effect of this, is that currently it’s not going to be possible to reliably get the GD-77 to connect to the duplex hotspot, if it has other modes also enabled.

I think even with the MD-380 which has 3 attempts to setup the call, its possible that the duplex hotspot may not be listening for DMR signals when the MD-380 does its 3 transmissions.


So Duplex operation of the hotspot should only be enabled if the hotspot is operating in a single mode.



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