Repeater System/Frequencies & Talk Groups

Update 1/16/2023: Some newer DMR radios have built in APRS capability.  SCHEART and NCPRN DMR networks do not support APRS data.  Operators must ensure they do not transmit APRS beacons on the networks since this transmission mode is NOT AUTHORIZED.  Thanks for everyone’s support.  

The DMR repeater system provides amateur radio operators access to a commercial level radio network whose operation is based on talk groups rather than just frequency channels.  Talk groups are unique digital identifiers that help route conversations through the network and into the user’s radio.  Authorized talk groups for use on SCHEART DMR are listed on the SCHEART web site at www.scheart.us  under the Digital drop-down tab.  DMR repeaters support two simultaneous voice/data conversations on a single frequency pair.  Each repeater is configured to support a full-time local talk group and a wide area talk group.  During normal operation, the SCHEART DMR network is integrated with the NCPRN network, and the wide area talk group (PRN) covers South Carolina, North Carolina, parts of adjacent states.

To support two simultaneous conversations on the same frequency, the repeater uses Time Division Multiple Access (TDMA) technology. With TDMA, conversations are divided into two time slots and the radio/repeater synchronizes their data depending on which time slot used.  In normal operation, SCHEART DMR is configured to support a local talk group (time slot 1) and a wide area (PRN time slot 2) talk group. Stations on the local talk group hear only local traffic while stations on the PRN talk group will hear any traffic on the multi-state network during normal operational mode.  Additional talk groups are available that share time slot 1 on a dynamic basis.  If a user keys one of the dynamic talk groups, that talk group shares time slot 1 with local.  The following dynamic talk groups are available: SC CALL, PRN CHAT 1 and PRN CHAT 2.

Operationally, stations should use the talk group that impacts the fewest repeaters for extended conversations.  The PRN talk group activates over 60 repeaters and is great for initial contact and for short conversations. If you make a contact with a station on PRN and plan to have an extended conversation, both stations should switch to a PRN Chat talk group.  Two Chat talk groups are provided: PRN Chat 1 and PRN Chat 2.  These are dynamic talk groups that do not have any repeaters connected full time.  Only stations that activate the PRN Chat talk group through a repeater will be connected.  Two stations may move their conversation from PRN to a Chat talk group or multiple stations may move.  In either case, moving frees up the PRN channel. When finished, select the Clear Timeslot talk group and key your transceiver.  This action will cancel the talk group on the repeater.  If you fail to clear the time slot, the connection remains active for 15 minutes or longer if there is activity. 

CLEAR TIME SLOT- This talk group is used to clear your connection when using PRN CHAT 1 or 2.  If you do not clear these talk groups after you connect, the time slot will continue to carry their audio for 15 minutes and can interrupt conversations on the local talk group.

ECHOTEST- This talk group provides an option for users to test their signal and audio quality into the system.  A user pushes PTT and when they receive a talk permit tone, can send a voice message.  After releasing the PTT switch, the system will echo back your transmission.  This talk group may be unavailable when the system is in the emergency operation mode.

SCHEART DMR has two operational modes: Normal Operations and Emergency Operations.  During Normal Operations, all talk groups are available to stations and the PRN talk group covers multiple states.  SCHEART also has a special talk group called SC Call. The SC Call talk group uses time slot 1 and is similar to PRN talk group except it only operates on SCHEART repeaters.  SC Call traffic is broadcast on all SCHEART repeaters with the following exception: 1) the repeater is busy with Local talk group traffic or 2) the repeater is busy with PRN CHAT 1 or 2 traffic.  A hold off timer prevents SC Call traffic from interrupting ongoing conversations for 5 minutes on those repeaters.  SC Call talk group is to establish an initial contact and short conversations within the state.  For long conversations, please move to either PRN Chat 1 or PRN Chat 2 talk group.

During exercises and incident responses, the DMR system may revert to emergency operational mode (EOM).  Under the EOM, the PRN talk group is restricted to SCHEART repeaters.  No traffic is routed to repeaters outside of South Carolina.  The Local Talk Group is not affected but normal dynamic talk groups outside the state are suspended.  Additional talk groups to support mutual aid communications are activated along with a data channel.   Other talk groups may be established by the network administrators as necessary to support the response.  The DMR system will remain open but stations supporting the incident should be provided priority access.  The following two tables describe talk groups in both modes:

Normal Operational Mode:

Talk Group Name                       Talk Group Code                                      Time Slot

Emergency Operational Mode

Talk Group Name                       Talk Group Code                                      Time Slot

* PRN wide area traffic restricted to repeaters within SC.

** Dynamic PTT group

SCHEART  DMR Repeater Information – All repeaters use Color code = 1 

 “- SCHEART partner repeater

Emergency Trailers will only be available on the network during exercises or a response event. They are not normally connected to the network.