diff --git a/Modem.cpp b/Modem.cpp
index cff03e4..a70cd76 100644
--- a/Modem.cpp
+++ b/Modem.cpp
@@ -784,6 +784,8 @@ bool CModem::setConfig()
 		buffer[3U] |= 0x02U;
 	if (m_pttInvert)
 		buffer[3U] |= 0x04U;
+	if (m_duplex)
+		buffer[3U] |= 0x80U;
 
 	buffer[4U] = 0x00U;
 	if (m_dstarEnabled)