diff --git a/src/ftdiJtagBitbang.cpp b/src/ftdiJtagBitbang.cpp
index 340ebe4..1b66699 100644
--- a/src/ftdiJtagBitbang.cpp
+++ b/src/ftdiJtagBitbang.cpp
@@ -23,6 +23,7 @@
 #include <map>
 #include <vector>
 #include <string>
+#include <stdexcept>
 
 #include "ftdiJtagBitbang.hpp"
 #include "ftdipp_mpsse.hpp"
@@ -44,18 +45,38 @@ FtdiJtagBitBang::FtdiJtagBitBang(const FTDIpp_MPSSE::mpsse_bit_config &cable,
 			const jtag_pins_conf_t *pin_conf, string dev, const std::string &serial,
 			uint32_t clkHZ, bool verbose):
 			FTDIpp_MPSSE(cable, dev, serial, clkHZ, verbose), _bitmode(0),
-			_curr_tms(0)
+			_curr_tms(0), _rx_size(0)
 {
+	unsigned char *ptr;
+
 	_tck_pin = (1 << pin_conf->tck_pin);
 	_tms_pin = (1 << pin_conf->tms_pin);
 	_tdi_pin = (1 << pin_conf->tdi_pin);
 	_tdo_pin = (1 << pin_conf->tdo_pin);
 
-	_buffer_size = 512;  // TX Fifo size
+	/* store FTDI TX Fifo size */
+	if (_pid == 0x6001)  // FT232R
+		_rx_size = 256;
+	else if (_pid == 0x6015)  // FT231X
+		_rx_size = 512;
+	else
+		_rx_size = _buffer_size;
+
+	/* RX Fifo size (rx: USB -> FTDI)
+	 * is 128 or 256 Byte and MaxPacketSize ~= 64Byte
+	 * but we let subsystem (libftdi, libusb, linux)
+	 * sending with the correct size -> this reduce hierarchical calls
+	 */
+	_buffer_size = 4096;
+
+	/* _buffer_size has changed -> resize buffer */
+	ptr = (unsigned char *)realloc(_buffer, sizeof(char) * _buffer_size);
+	if (!ptr)
+		throw std::runtime_error("_buffer realloc failed\n");
+	_buffer = ptr;
 
 	setClkFreq(_clkHZ);
 
-	_buffer = (unsigned char *)realloc(_buffer, sizeof(char) * _buffer_size);
 	init(1, _tck_pin | _tms_pin | _tdi_pin, BITMODE_BITBANG);
 	setBitmode(BITMODE_BITBANG);
 }
@@ -134,10 +155,11 @@ int FtdiJtagBitBang::writeTMS(uint8_t *tms, int len, bool flush_buffer)
 int FtdiJtagBitBang::writeTDI(uint8_t *tx, uint8_t *rx, uint32_t len, bool end)
 {
 	uint32_t iter;
-	if (len * 2 + 1 < (uint32_t)_buffer_size) {
+	uint32_t xfer_size = (rx) ? _rx_size : _buffer_size;
+	if (len * 2 + 1 < xfer_size) {
 		iter = len;
 	} else {
-		iter = _buffer_size >> 1;  // two tx / bit
+		iter = xfer_size >> 1;  // two tx / bit
 		iter = (iter / 8) * 8;
 	}
 
diff --git a/src/ftdiJtagBitbang.hpp b/src/ftdiJtagBitbang.hpp
index 8d8a523..d067584 100644
--- a/src/ftdiJtagBitbang.hpp
+++ b/src/ftdiJtagBitbang.hpp
@@ -58,21 +58,20 @@ class FtdiJtagBitBang : public JtagInterface, private FTDIpp_MPSSE {
 	 */
 	int get_buffer_size() override { return _buffer_size/8/2; }
 
-	bool isFull() override { return _nb_bit == 8*get_buffer_size();}
+	bool isFull() override { return _num == 8*get_buffer_size();}
 
 	int flush() override;
 
  private:
 	int write(uint8_t *tdo, int nb_bit);
 	int setBitmode(uint8_t mode);
-	uint8_t *_in_buf;
 
 	uint8_t _bitmode;
 	uint8_t _tck_pin; /*!< tck pin: 1 << pin id */
 	uint8_t _tms_pin; /*!< tms pin: 1 << pin id */
 	uint8_t _tdo_pin; /*!< tdo pin: 1 << pin id */
 	uint8_t _tdi_pin; /*!< tdi pin: 1 << pin id */
-	int _nb_bit;
 	uint8_t _curr_tms;
+	int _rx_size;
 };
 #endif