DP4800 won't transmit to DM4600 when using split frequencies.

[Q] We build our own semi-duplex interfaces with two DM4600 radios. We have the problem that the DP4800 won´t transmit to the RX DM4600. There is a forbidden sound. If we replace our own interface with a SLR5500 repeater, it works perfect. Is there any feedback from a slr 5500 in digital semi-duplex mode that a DM4600 is not supporting? [...]

[A] MOTOTRBO radios do not support the configuration of a direct mode channel that has different transmit and receive frequencies. When these frequencies are different, the channel is defined as a repeater channel and the radio expects to communicate with a repeater.

When transmitting via a repeater, the radio gets synchronisation signalling (and other stuff) back from the repeater which tells it that the transmission was received and can continue. The mobile is not able to produce this signaling and this is why the call fails in your case.