diff --git a/F0:F030,F042,F072/usbcan_gpio/Makefile b/F0:F030,F042,F072/usbcan_gpio/Makefile
index 3607fae..12e87d8 100644
--- a/F0:F030,F042,F072/usbcan_gpio/Makefile
+++ b/F0:F030,F042,F072/usbcan_gpio/Makefile
@@ -10,3 +10,6 @@ DEFINES := -DUSB2_16
 
 include ../makefile.f0
 include ../../makefile.stm32
+
+$(OBJDIR)/gpioproto.o: gpioproto.c $(VERSION_FILE)
+$(OBJDIR)/canproto.o: canproto.c $(VERSION_FILE)
diff --git a/F0:F030,F042,F072/usbcan_gpio/gpioproto.c b/F0:F030,F042,F072/usbcan_gpio/gpioproto.c
index afcbda2..b76d98f 100644
--- a/F0:F030,F042,F072/usbcan_gpio/gpioproto.c
+++ b/F0:F030,F042,F072/usbcan_gpio/gpioproto.c
@@ -104,7 +104,9 @@ static const char *cmd_parser(char *buf){
         case 'i':
             return setiface(buf);
         default:
-            return buf-1; // echo wrong data
+            // echo wrong data with terminating '\n'
+            SENDn(buf-1);
+            return NULL;
         }
     }
     // "short" commands
@@ -138,8 +140,9 @@ static const char *cmd_parser(char *buf){
 void GPIO_process(){
     char inbuff[MAXSTRLEN];
     int l = RECV(inbuff, MAXSTRLEN);
+    if(l == 0) return;
     if(l < 0) SEND("ERROR: USB buffer overflow or string was too long\n");
-    else if(l){
+    else{
         const char *ans = cmd_parser(inbuff);
         if(ans) SEND(ans);
     }
diff --git a/F0:F030,F042,F072/usbcan_gpio/ringbuffer.c b/F0:F030,F042,F072/usbcan_gpio/ringbuffer.c
index 24c8b61..389c9e7 100644
--- a/F0:F030,F042,F072/usbcan_gpio/ringbuffer.c
+++ b/F0:F030,F042,F072/usbcan_gpio/ringbuffer.c
@@ -19,6 +19,8 @@
 #include <stm32f0.h>
 #include "ringbuffer.h"
 
+#define CHK(b)  do{if(!b) return -1;}while(0)
+
 static int datalen(ringbuffer *b){
     if(b->tail >= b->head) return (b->tail - b->head);
     else return (b->length - b->head + b->tail);
@@ -26,7 +28,9 @@ static int datalen(ringbuffer *b){
 
 // stored data length
 int RB_datalen(ringbuffer *b){
-    if(!b || b->busy) return -1;
+    CHK(b);
+    if(0 == datalen(b)) return 0; // don't block for empty RO operations
+    if(b->busy) return -1;
     b->busy = 1;
     int l = datalen(b);
     b->busy = 0;
@@ -34,7 +38,7 @@ int RB_datalen(ringbuffer *b){
 }
 
 static int hasbyte(ringbuffer *b, uint8_t byte){
-    if(!b || b->head == b->tail) return -1; // no data in buffer
+    if(b->head == b->tail) return -1; // no data in buffer
     int startidx = b->head;
     if(b->head > b->tail){ //
         for(int found = b->head; found < b->length; ++found)
@@ -53,7 +57,8 @@ static int hasbyte(ringbuffer *b, uint8_t byte){
  * @return index if found, -1 if none or busy
  */
 int RB_hasbyte(ringbuffer *b, uint8_t byte){
-    if(!b || b->busy) return -1;
+    CHK(b);
+    if(b->busy) return -1;
     b->busy = 1;
     int ret = hasbyte(b, byte);
     b->busy = 0;
@@ -91,7 +96,10 @@ static int read(ringbuffer *b, uint8_t *s, int len){
  * @return bytes read or -1 if busy
  */
 int RB_read(ringbuffer *b, uint8_t *s, int len){
-    if(!b || b->busy || !s || len < 1) return -1;
+    CHK(b);
+    if(!s || len < 1) return -1;
+    if(0 == datalen(b)) return 0;
+    if(b->busy) return -1;
     b->busy = 1;
     int r = read(b, s, len);
     b->busy = 0;
@@ -124,7 +132,10 @@ static int readto(ringbuffer *b, uint8_t byte, uint8_t *s, int len){
  * @return amount of bytes written (negative, if len<data in buffer or buffer is busy)
  */
 int RB_readto(ringbuffer *b, uint8_t byte, uint8_t *s, int len){
-    if(!b || b->busy) return -1;
+    CHK(b);
+    if(!s || len < 1) return -1;
+    if(0 == datalen(b)) return 0;
+    if(b->busy) return -1;
     b->busy = 1;
     int n = 0;
     if(s && len > 0){
@@ -137,7 +148,9 @@ int RB_readto(ringbuffer *b, uint8_t byte, uint8_t *s, int len){
 }
 
 int RB_datalento(ringbuffer *b, uint8_t byte){
-    if(!b || b->busy) return -1;
+    CHK(b);
+    if(0 == datalen(b)) return 0;
+    if(b->busy) return -1;
     b->busy = 1;
     int n = lento(b, byte);
     b->busy = 0;
@@ -167,7 +180,10 @@ static int write(ringbuffer *b, const uint8_t *str, int l){
  * @return amount of bytes written or -1 if busy
  */
 int RB_write(ringbuffer *b, const uint8_t *str, int l){
-    if(!b || b->busy || !str || l < 1) return -1;
+    CHK(b);
+    if(!str || l < 1) return -1;
+    if(b->length - datalen(b) < 2) return 0;
+    if(b->busy) return -1;
     b->busy = 1;
     int w = write(b, str, l);
     b->busy = 0;
@@ -176,10 +192,12 @@ int RB_write(ringbuffer *b, const uint8_t *str, int l){
 
 // just delete all information in buffer `b`
 int RB_clearbuf(ringbuffer *b){
-    if(!b || b->busy) return -1;
+    CHK(b);
+    if(b->busy) return -1;
     b->busy = 1;
     b->head = 0;
     b->tail = 0;
+    bzero(b->data, b->length);
     b->busy = 0;
     return 1;
 }
diff --git a/F0:F030,F042,F072/usbcan_gpio/usb_dev.c b/F0:F030,F042,F072/usbcan_gpio/usb_dev.c
index 249db9f..6d015cf 100644
--- a/F0:F030,F042,F072/usbcan_gpio/usb_dev.c
+++ b/F0:F030,F042,F072/usbcan_gpio/usb_dev.c
@@ -119,13 +119,15 @@ static void rxtx_handler(){
     }
 }
 
-// clear IN/OUT buffers on connection
-static void clearbufs(uint8_t ifno){
+static void clearRbuf(uint8_t ifno){
     uint32_t T0 = Tms;
     while(Tms - T0 < 10){ // wait no more than 10ms
         if(1 == RB_clearbuf((ringbuffer*)&rbin[ifno])) break;
     }
-    T0 = Tms;
+}
+
+static void clearTbuf(uint8_t ifno){
+    uint32_t T0 = Tms;
     while(Tms - T0 < 10){
         if(1 == RB_clearbuf((ringbuffer*)&rbout[ifno])) break;
     }
@@ -144,7 +146,8 @@ void clstate_handler(uint8_t ifno, uint16_t val){
     CDCready[ifno] = val; // CONTROL_DTR | CONTROL_RTS -> interface connected; 0 -> disconnected
     lastdsz[ifno] = -1;
     if(val){
-        clearbufs(ifno);
+        clearRbuf(ifno);
+        clearTbuf(ifno);
         EP_reset(EPNO(ifno));
         // usart_start(ifno);
     }//else usart_stop(ifno); // turn of USART (if it is @ this interface)
@@ -306,7 +309,7 @@ int USB_receive(uint8_t ifno, uint8_t *buf, int len){
     if(!CDCready[ifno]) return 0;
     chkin(ifno); // rxtx_handler could leave last message unwritten if buffer was busy
     if(bufovrfl[ifno]){
-        while(1 != RB_clearbuf((ringbuffer*)&rbin[ifno])); // run watchdog in case of problems
+        clearRbuf(ifno);
         bufovrfl[ifno] = 0;
         return -1;
     }
@@ -325,7 +328,7 @@ int USB_receivestr(uint8_t ifno, char *buf, int len){
     if(!CDCready[ifno]) return 0;
     chkin(ifno); // rxtx_handler could leave last message unwritten if buffer was busy
     if(bufovrfl[ifno]){
-        while(1 != RB_clearbuf((ringbuffer*)&rbin[ifno]));
+        clearRbuf(ifno);
         bufovrfl[ifno] = 0;
         return -1;
     }
@@ -333,7 +336,7 @@ int USB_receivestr(uint8_t ifno, char *buf, int len){
     if(l < 1){
         if((rbin[ifno].length <= RB_datalen((ringbuffer*)&rbin[ifno]) + 1) ||
             (RB_datalento((ringbuffer*)&rbin[ifno], '\n') > len - 1)){ // buffer is full but no '\n' found or string too long
-            while(1 != RB_clearbuf((ringbuffer*)&rbin[ifno]));
+            clearRbuf(ifno);
             return -1;
         }
         return 0;
diff --git a/F0:F030,F042,F072/usbcan_gpio/usb_lib.c b/F0:F030,F042,F072/usbcan_gpio/usb_lib.c
index 3bdfd8c..8ed4e0e 100644
--- a/F0:F030,F042,F072/usbcan_gpio/usb_lib.c
+++ b/F0:F030,F042,F072/usbcan_gpio/usb_lib.c
@@ -301,10 +301,14 @@ int EP_Init(uint8_t number, uint8_t type, uint16_t txsz, uint16_t rxsz, void (*f
 }
 
 // refresh EP after interface reconnected
-void EP_reset(uint8_t number){
-    if(number >= STM32ENDPOINTS) return;
-    USB->EPnR[number] ^= USB_EPnR_STAT_RX | USB_EPnR_STAT_TX;
-    USB_BTABLE->EP[number].USB_COUNT_TX = 0;
+void EP_reset(uint8_t epno){
+    if(epno >= STM32ENDPOINTS) return;
+    // keep DTOGs (don't write 1 to them), clear CTR (write 0 to them)
+    //      and set STAT to VALID (write 1 where was 0)
+    uint16_t epstatus = KEEP_DTOG(USB->EPnR[epno]);
+    USB->EPnR[epno] = (epstatus & ~(USB_EPnR_CTR_TX|USB_EPnR_CTR_RX)) ^
+                        (USB_EPnR_STAT_RX | USB_EPnR_STAT_TX);
+    USB_BTABLE->EP[epno].USB_COUNT_TX = 0;
 }
 
 // standard IRQ handler
diff --git a/F0:F030,F042,F072/usbcan_gpio/usb_lib.h b/F0:F030,F042,F072/usbcan_gpio/usb_lib.h
index 18ede16..1dfb3a8 100644
--- a/F0:F030,F042,F072/usbcan_gpio/usb_lib.h
+++ b/F0:F030,F042,F072/usbcan_gpio/usb_lib.h
@@ -345,7 +345,7 @@ int EP_Init(uint8_t number, uint8_t type, uint16_t txsz, uint16_t rxsz, void (*f
 void EP_WriteIRQ(uint8_t number, const uint8_t *buf, uint16_t size);
 void EP_Write(uint8_t number, const uint8_t *buf, uint16_t size);
 int EP_Read(uint8_t number, uint8_t *buf);
-void EP_reset(uint8_t number);
+void EP_reset(uint8_t epno);
 
 // could be [re]defined in usb_dev.c
 extern void usb_class_request(config_pack_t *packet, uint8_t *data, uint16_t datalen);
diff --git a/F0:F030,F042,F072/usbcan_gpio/usbcangpio.bin b/F0:F030,F042,F072/usbcan_gpio/usbcangpio.bin
index 8716eef..1a75ca5 100755
Binary files a/F0:F030,F042,F072/usbcan_gpio/usbcangpio.bin and b/F0:F030,F042,F072/usbcan_gpio/usbcangpio.bin differ
diff --git a/F0:F030,F042,F072/usbcan_gpio/version.inc b/F0:F030,F042,F072/usbcan_gpio/version.inc
index b49f3d3..789476e 100644
--- a/F0:F030,F042,F072/usbcan_gpio/version.inc
+++ b/F0:F030,F042,F072/usbcan_gpio/version.inc
@@ -1,2 +1,2 @@
-#define BUILD_NUMBER "81"
+#define BUILD_NUMBER "85"
 #define BUILD_DATE "2026-03-07"
diff --git a/F3:F303/InterfaceBoard/TODO b/F3:F303/InterfaceBoard/TODO
new file mode 100644
index 0000000..8fdc6eb
--- /dev/null
+++ b/F3:F303/InterfaceBoard/TODO
@@ -0,0 +1 @@
+add fixes to ringbuffer and make EP_reset @ each reconnection (like in F0/usbcan_gpio).