diff --git a/F3:F303/InterfaceBoard/Debug.c b/F3:F303/InterfaceBoard/Debug.c
new file mode 100644
index 0000000..b19f79a
--- /dev/null
+++ b/F3:F303/InterfaceBoard/Debug.c
@@ -0,0 +1,66 @@
+/*
+ * This file is part of the multiiface project.
+ * Copyright 2026 Edward V. Emelianov <edward.emelianoff@gmail.com>.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <string.h>
+#include "Debug.h"
+#include "hardware.h" // Config_mode
+#include "ringbuffer.h"
+#include "usb_dev.h"
+
+// asinchronous debugging
+// DBG(str): add filename, line, string message `str` and '\n'
+// DBGs(str): only add `str`, without '\n'
+// DBGn(): add just '\n'
+
+#ifdef EBUG
+
+#define DBGBUFSZ   (1024)
+
+static uint8_t buf[DBGBUFSZ];
+static ringbuffer dbgrb = {.data = buf, .length = DBGBUFSZ};
+
+void debug_message_text(const char *str){
+    if(!Config_mode) return;
+    RB_write(&dbgrb, (const uint8_t*)str, strlen(str));
+}
+
+void debug_message_char(char ch){
+    if(!Config_mode) return;
+     RB_write(&dbgrb, (const uint8_t*)&ch, 1);
+}
+
+void debug_newline_only(){
+    if(!Config_mode) return;
+    char nl = '\n';
+    RB_write(&dbgrb, (const uint8_t*)&nl, 1);
+}
+
+void print_debug_messages(){
+    if(!Config_mode) return;
+    uint8_t rcvbuf[256];
+    do{
+        int n = RB_readto(&dbgrb, '\n', rcvbuf, 256);
+        if(n == 0) break;
+        else if(n < 0) n = -n; // partial string: longer than 256 bytes
+        USB_send(ICFG, rcvbuf, n);
+    }while(1);
+}
+
+#endif
+
+
diff --git a/F3:F303/InterfaceBoard/Debug.h b/F3:F303/InterfaceBoard/Debug.h
new file mode 100644
index 0000000..02ea009
--- /dev/null
+++ b/F3:F303/InterfaceBoard/Debug.h
@@ -0,0 +1,42 @@
+/*
+ * This file is part of the multiiface project.
+ * Copyright 2026 Edward V. Emelianov <edward.emelianoff@gmail.com>.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "usb_dev.h"
+
+// debugging messages on config interface
+#ifdef EBUG
+#define STR_HELPER(s)   #s
+#define STR(s)          STR_HELPER(s)
+#define DBG(str)        do{debug_message_text(__FILE__ " (L" STR(__LINE__) "): " str); debug_newline_only();}while(0)
+#define DBGs(str)       debug_message_text(str)
+#define DBGch(ch)       debug_message_char(ch)
+#define DBGn()          debug_newline_only()
+#define DBGpri()        print_debug_messages();
+#else
+#define DBG(str)
+#define DBGs(str)
+#define DBGn()
+#define DBGpri()
+#endif
+
+void debug_message_text(const char *str);
+void debug_message_char(char ch);
+void debug_newline_only();
+void print_debug_messages();