GB7HT is an MMDVM UHF DMR Repeater, situated in Ashington, Northumberland – IO95FE, owned and operated by Chris G4NAB. Follow the dashboard here.

The original hardware was the former F5ZLW repeater, with some changes, notably the frequencies of operation and the split between Transmit and Receive, and a re-tuned Procomm Duplexer for those frequencies.

The channel is DVU59 Tx 439.7375 MHz and Rx 430.7375 MHz. The ColorCode is 10 and the DMR-Gateway is active.

Here any resemblance to FM ends, and it is important to understand that a DMR repeater transmits two streams of digital data, in the form of time-slices to each of two TimeSlots sequentially. This is what is known as TDMA or Time Division Multiple Access. So Time Slot 1 and Time Slot 2 can each be considered as individual repeaters.

To use one of these Time Slots, a DMR Radio has to be programmed first of all with the reverse frequencies of the repeater. It also needs the ColorCode to enable full access to the repeater functionality. However this is not the end. To enable a transmission, the date stream needs a path to travel, a destination code. These destination codes are commonly called TalkGroups, and generally follow a structure and a protocol as to which of the two slots is the normal.

So with the frequencies, the colorcode, the TalkGroup and the TimeSlot, this new channel needs to be compiled into a usable zone to be displayed in the window of the radio, sometimes called a terminal. It follows that if other TalkGroups are required, then the simple course of action is to copy and paste the first channel into a new channel and change the name and the TalkGroup. Added to the existing zone, the radio now contains two or more channels on the same repeater, on one or other of the two time slots.

When a transmission is made to a DMR Repeater, it is in bursts, that the repeater interprets as GOOD data, so permits further data to pass, or BAD data, in which case it refuses. These acknowledgements take very little time, and it follows that only GOOD data will pass over the repeater. This is known as Handshaking. A good handshake is essential to permit communication.

If the TalkGroup is not already set in the repeater, then the DMR Server checks that it is a valid TalkGroup and creates a path. A TalkGroup set on the repeater is known as ‘Fixed’ and one that is not already there is known as ‘Dynamic’, requiring user activation.

GB7HT adheres to this protocol, in that certain TalkGroups are ‘Fixed’, but is open to use of ‘Dynamic’ TalkGroups also. Some are fixed on TimeSlot 1 and some on TimeSlot2. For the listener, this means that there will always be transmissions on the repeater if any of the Fixed TalkGroups are in use.

The DMR-Gateway on GB7HT permits multiple networks, and in this case both DMR-BrandMeister and DMR+-Phoenix are operational, and XLX reflectors are available.

In the case of the DMR Gateway it is essential to understand that in any transmission of the repeater, the first transmission is the one accepted. This is applicable to both Time Slots. What this means is that if someone wants to activate TimeSlot 1, he needs to be aware that there may a transmission already there from another network.

It is best explained in the following way.

BrandMeister has a centre of activity in the UK on TalkGroup 2350 (UK Wide), that is normally on TimeSlot 2. On GB7HT this is so. However in the Tyne and Wear group of repeaters, there is a TalkGroup 23561 used for local activity, which on all the repeaters is on TimeSlot1.

DMR+ Phoenix has a centre of activity on TalkGroup 235 that is invariably on TimeSlot 1 on all repeaters. However DMR BrandMeister also has a TalkGroup 235 (UK Wide – International) that is also invariably on TimeSlot1. They are not linked. However the MDMDVM cannot decide for itself the destination of a call on 235, as in the case of GB7HT, TalkGroup 235 will go only to the BrandMeister network.

To permit the use of either network by a user, some of these TalkGroups need to be rewritten, in such a way that GB7HT knows where to direct the call. For BrandMeister this is rather simple, in that the TalkGroups remain unchanged. However to use the DMR+ Phoenix network, the TalkGroup number has to be summed with 8000. So TalkGroup 235 becomes 8235 in the radio. TalkGroup 80 becomes TalkGroup 8080 in the radio. The repeater software does the rest.

The XLX network is more complicated. It requires research into what realm the user wants to explore. However the radio needs to have a dedicated channel for the repeater, with TalkGroup 6 programmed on Time Slot 2. In the case of GB7HT, XLX Reflector 247 Module E is the start point. More on this another time.

Take a look at the table below for an example method to program the radio – CodePlug.

TimeSlot 1TimeSlot 2
BrandMeister235 (Fixed)2350 (Fixed)
23561 (Fixed)
DMR+ Phoenix8235 (235) (Fixed)8860 (860) (Fixed)
8080 (80) 8081 (81)
8082 (82) 8083 (83)
8084 (84)
Talk Groups for a code plug on GB7HT

This is not a complete picture, as there are other BrandMeister TalkGroups of 4 Digits (that should be on TimeSlot 2), and TalkGroups of 5 Digits like 23561 (that should be on TimeSlot1). To be able to use them, the user has to check the activity on the TimeSlot, as if they are in use then the new choice of TalkGroup will not be possible until the activity is ceased.

For those more experienced on DMR, BrandMeister TG 91 is unavailable. The oversubscription of this channel does not permit sufficient control of connection time.

SInce I wrote this article I have added an experimental portal to Fusion, using the DMR2YSF on the DMR-Gateway and linked it to FCS00210 – Florida, in an effort to communicate with my brother in Orlando. to be frank, I don’t yet know what I’m doing, but I’ll grasp Fusion eventually, it’s just worse than DMR for networks and I don’t think they know what they are doing either.

Temporary Loom test prior to installation
Raspberry Pi & MMDVM prior to removal of the alarm board

View the live dashboard of GB7HT here.

GB7HT Complete

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