diff --git a/Globals.h b/Globals.h
index d68e2c5..a44e888 100644
--- a/Globals.h
+++ b/Globals.h
@@ -97,6 +97,12 @@ const uint16_t RX_BLOCK_SIZE = 2U;
 const uint16_t TX_RINGBUFFER_SIZE = 500U;
 const uint16_t RX_RINGBUFFER_SIZE = 600U;
 
+#if defined(STM32F105xC)
+const uint16_t TX_BUFFER_LEN = 2000U;
+#else
+const uint16_t TX_BUFFER_LEN = 4000U;
+#endif
+
 extern MMDVM_STATE m_modemState;
 
 extern bool m_dstarEnable;
diff --git a/NXDNTX.cpp b/NXDNTX.cpp
index 2601101..45d8ffa 100644
--- a/NXDNTX.cpp
+++ b/NXDNTX.cpp
@@ -45,7 +45,7 @@ const uint8_t NXDN_PREAMBLE[] = {0x57U, 0x75U, 0xFDU};
 const uint8_t NXDN_SYNC = 0x5FU;
 
 CNXDNTX::CNXDNTX() :
-m_buffer(4000U),
+m_buffer(TX_BUFFER_LEN),
 m_modFilter(),
 m_sincFilter(),
 m_modState(),
diff --git a/P25TX.cpp b/P25TX.cpp
index 7cc75d9..9db1747 100644
--- a/P25TX.cpp
+++ b/P25TX.cpp
@@ -43,7 +43,7 @@ const q15_t P25_LEVELD = -1260;
 const uint8_t P25_START_SYNC = 0x77U;
 
 CP25TX::CP25TX() :
-m_buffer(4000U),
+m_buffer(TX_BUFFER_LEN),
 m_modFilter(),
 m_lpFilter(),
 m_modState(),
diff --git a/SerialRB.h b/SerialRB.h
index 22667d5..a2ccb8b 100644
--- a/SerialRB.h
+++ b/SerialRB.h
@@ -32,7 +32,11 @@ Boston, MA  02110-1301, USA.
 #include <Arduino.h>
 #endif
 
+#if defined(STM32F105xC)
+const uint16_t SERIAL_RINGBUFFER_SIZE = 370U;
+#else
 const uint16_t SERIAL_RINGBUFFER_SIZE = 1000U;
+#endif
 
 class CSerialRB {
 public:
diff --git a/YSFTX.cpp b/YSFTX.cpp
index 5790334..4f0400d 100644
--- a/YSFTX.cpp
+++ b/YSFTX.cpp
@@ -44,7 +44,7 @@ const uint8_t YSF_END_SYNC   = 0xFFU;
 const uint8_t YSF_HANG       = 0x00U;
 
 CYSFTX::CYSFTX() :
-m_buffer(4000U),
+m_buffer(TX_BUFFER_LEN),
 m_modFilter(),
 m_modState(),
 m_poBuffer(),