diff --git a/TNC/PortInterface.hpp b/TNC/PortInterface.hpp
index 3607152..60109b1 100644
--- a/TNC/PortInterface.hpp
+++ b/TNC/PortInterface.hpp
@@ -11,7 +11,7 @@
 
 namespace mobilinkd { namespace tnc {
 
-const uint32_t TX_BUFFER_SIZE = 512;
+const uint32_t TX_BUFFER_SIZE = 64;     // Cannot be larger that USB_FS_MAX_PACKET_SIZE.
 extern uint8_t TxBuffer[TX_BUFFER_SIZE];
 
 /**