diff --git a/.gitignore b/.gitignore
index 84b8e8319..79f2a8058 100644
--- a/.gitignore
+++ b/.gitignore
@@ -67,3 +67,7 @@ PVS-Studio.log
 
 # JS packages
 node_modules/
+
+# cli_perf script output in case of errors
+/block.bin
+/return_block.bin
diff --git a/scripts/serial_cli_perf.py b/scripts/serial_cli_perf.py
index 0fed7c393..0db53dd6a 100644
--- a/scripts/serial_cli_perf.py
+++ b/scripts/serial_cli_perf.py
@@ -32,16 +32,25 @@ def main():
 
     bytes_to_send = args.length
     block_size = 1024
+    success = True
     while bytes_to_send:
         actual_size = min(block_size, bytes_to_send)
         # can't use 0x03 because that's ASCII ETX, or Ctrl+C
-        block = bytes([randint(4, 255) for _ in range(actual_size)])
+        # block = bytes([randint(4, 255) for _ in range(actual_size)])
+        block = bytes([4 + (i // 64) for i in range(actual_size)])
 
         port.write(block)
         return_block = port.read(actual_size)
 
         if return_block != block:
-            logger.error("Incorrect block received")
+            with open("block.bin", "wb") as f:
+                f.write(block)
+            with open("return_block.bin", "wb") as f:
+                f.write(return_block)
+
+            logger.error("Incorrect block received. Saved to `block.bin' and `return_block.bin'.")
+            logger.error(f"{bytes_to_send} bytes left. Aborting.")
+            success = False
             break
 
         bytes_to_send -= actual_size
@@ -49,7 +58,8 @@ def main():
     end_time = time()
     delta = end_time - start_time
     speed = args.length / delta
-    print(f"Speed: {speed/1024:.2f} KiB/s")
+    if success:
+        print(f"Speed: {speed/1024:.2f} KiB/s")
 
     port.write(b"\x03")  # Ctrl+C
     port.close()