Pyramid has a clever protocol for multiple units at a scene, but it is not 100% infallible.
This is from the service manual:
When the SVR-200 is first activated, it will transmit a short “lock tone” that alerts the user that the system is
functioning. It will then assume the priority status and be ready to repeat any base-to-portable or portable-to-base
transmissions. If another unit arrives on scene and is activated, it too will transmit the “lock tone”; when the first
SVR-200 detects the lock tone from the second unit, it will increment a “priority counter” and will no longer repeat
any transmissions. The recently arrived unit will be the priority repeater, and the first unit will be 1 count away from
priority. This process will continue for each unit that arrives at the site, creating a priority hierarchy for up to 256
vehicles, each with a unique count and only one unit at priority status. The SVR-200 will not transmit it’s lock tone
if the radio channel is busy when first enabled. It will wait in non-priority status until all transmissions cease, then
send its lock tone and become the priority unit.
Even though the other SVR-200s are not at priority status, they will continue to monitor the channel for activity.
If the priority unit were to leave the scene or become disabled, the other units will detect the condition to repeat
and determine that there is no priority unit repeating the transmission. They will then begin to decrement their
priority counters until one of them reaches the priority status and begins repeating the transmission. Since the
SVR-200s are all at different counts, only one will reach priority status and begin transmitting. The other units will
sense the new priority repeater and cease counting down, preserving the priority hierarchy.
If another unit were to arrive from a different scene and it is still the active priority, there will be two active
repeaters on the air when a condition to repeat exists. When one of the SVR-200s unkeys to check for handheld
activity, it will detect the presence of the other active SVR-200 and increment it’s priority counter and cease
transmission. This is the self clearing mode to prevent radio collisions.
If the handheld operator is out of the vehicle and their partner still in the vehicle were to key the mobile radio
using the local mic, the SVR-200 will detect the local PTT and repeat the transmission to the other handhelds so
that both sides of the conversation will be heard by everyone on the link. The local mic repeat function can be
enabled/disabled via the PC software.
The SVR-200 also has a local receive audio speaker jack that enables the person in the vehicle to monitor
portable-to-base transmissions that are being repeated through the mobile.
If the users wish to communicate portable-to-portable without accessing the mobile repeater, they may transmit
on the same frequency without CTCSS (or a different CTCSS); the SVR-200 only responds to carrier and proper
tone from the handhelds.