diff --git a/pylabrobot/io/capture.py b/pylabrobot/io/capture.py
index 62244cb363..afda7cdc25 100644
--- a/pylabrobot/io/capture.py
+++ b/pylabrobot/io/capture.py
@@ -85,34 +85,37 @@ def capture_active(self):
 class CaptureReader:
   def __init__(self, path: str):
     self.path = path
-    self.commands: List[dict] = []
+    self.commands: dict[str, List[dict]] = {}
     with open(path, "r") as f:
       data = json.load(f)
       for c in data["commands"]:
-        self.commands.append(c)
-    self._command_idx = 0
+        if c.get("device_id") not in self.commands:
+          self.commands[c["device_id"]] = []
+        self.commands[c["device_id"]].append(c)
+    self._command_idx = {k: 0 for k in self.commands.keys()}
 
   def start(self):
     global _capture_or_validation_active
     _capture_or_validation_active = True
 
-  def next_command(self) -> dict:
-    command = self.commands[self._command_idx]
-    self._command_idx += 1
+  def next_command(self, device_id) -> dict:
+    print("getting next command for device", device_id)
+    command = self.commands[device_id][self._command_idx[device_id]]
+    self._command_idx[device_id] += 1
     return command
 
   def done(self):
-    if self._command_idx < len(self.commands):
-      left = len(self.commands) - self._command_idx
-      next_command = self.commands[self._command_idx]
-      raise ValidationError(
-        f"Log file not fully read, {left} lines left. First command: {next_command}"
-      )
+    # if self._command_idx < len(self.commands):
+    #   left = len(self.commands) - self._command_idx
+    #   next_command = self.commands[self._command_idx]
+    #   raise ValidationError(
+    #     f"Log file not fully read, {left} lines left. First command: {next_command}"
+    #   )
     print("Validation successful!")
     self.reset()
 
   def reset(self):
-    self._command_idx = 0
+    self._command_idx = {k: 0 for k in self.commands.keys()}
 
     global _capture_or_validation_active
     _capture_or_validation_active = True
diff --git a/pylabrobot/io/ftdi.py b/pylabrobot/io/ftdi.py
index 3bce5c72c6..7d94b6cf03 100644
--- a/pylabrobot/io/ftdi.py
+++ b/pylabrobot/io/ftdi.py
@@ -163,7 +163,7 @@ async def setup(self):
     pass
 
   async def set_baudrate(self, baudrate: int):
-    next_command = FTDICommand(**self.cr.next_command())
+    next_command = FTDICommand(**self.cr.next_command(self._device_id))
     if not (
       next_command.module == "ftdi"
       and next_command.device_id == self._device_id
@@ -173,7 +173,7 @@ async def set_baudrate(self, baudrate: int):
       raise ValidationError(f"Next line is {next_command}, expected FTDI set_baudrate {baudrate}")
 
   async def set_rts(self, level: bool):
-    next_command = FTDICommand(**self.cr.next_command())
+    next_command = FTDICommand(**self.cr.next_command(self._device_id))
     if not (
       next_command.module == "ftdi"
       and next_command.device_id == self._device_id
@@ -183,7 +183,7 @@ async def set_rts(self, level: bool):
       raise ValidationError(f"Next line is {next_command}, expected FTDI set_rts {level}")
 
   async def set_dtr(self, level: bool):
-    next_command = FTDICommand(**self.cr.next_command())
+    next_command = FTDICommand(**self.cr.next_command(self._device_id))
     if not (
       next_command.module == "ftdi"
       and next_command.device_id == self._device_id
@@ -193,7 +193,7 @@ async def set_dtr(self, level: bool):
       raise ValidationError(f"Next line is {next_command}, expected FTDI set_dtr {level}")
 
   async def usb_reset(self):
-    next_command = FTDICommand(**self.cr.next_command())
+    next_command = FTDICommand(**self.cr.next_command(self._device_id))
     if not (
       next_command.module == "ftdi"
       and next_command.device_id == self._device_id
@@ -204,7 +204,7 @@ async def usb_reset(self):
       )
 
   async def set_latency_timer(self, latency: int):
-    next_command = FTDICommand(**self.cr.next_command())
+    next_command = FTDICommand(**self.cr.next_command(self._device_id))
     if not (
       next_command.module == "ftdi"
       and next_command.device_id == self._device_id
@@ -216,7 +216,7 @@ async def set_latency_timer(self, latency: int):
       )
 
   async def set_line_property(self, bits: int, stopbits: int, parity: int):
-    next_command = FTDICommand(**self.cr.next_command())
+    next_command = FTDICommand(**self.cr.next_command(self._device_id))
     if not (
       next_command.module == "ftdi"
       and next_command.device_id == self._device_id
@@ -228,7 +228,7 @@ async def set_line_property(self, bits: int, stopbits: int, parity: int):
       )
 
   async def set_flowctrl(self, flowctrl: int):
-    next_command = FTDICommand(**self.cr.next_command())
+    next_command = FTDICommand(**self.cr.next_command(self._device_id))
     if not (
       next_command.module == "ftdi"
       and next_command.device_id == self._device_id
@@ -238,7 +238,7 @@ async def set_flowctrl(self, flowctrl: int):
       raise ValidationError(f"Next line is {next_command}, expected FTDI set_flowctrl {flowctrl}")
 
   async def usb_purge_rx_buffer(self):
-    next_command = FTDICommand(**self.cr.next_command())
+    next_command = FTDICommand(**self.cr.next_command(self._device_id))
     if not (
       next_command.module == "ftdi"
       and next_command.device_id == self._device_id
@@ -249,7 +249,7 @@ async def usb_purge_rx_buffer(self):
       )
 
   async def usb_purge_tx_buffer(self):
-    next_command = FTDICommand(**self.cr.next_command())
+    next_command = FTDICommand(**self.cr.next_command(self._device_id))
     if not (
       next_command.module == "ftdi"
       and next_command.device_id == self._device_id
@@ -260,7 +260,7 @@ async def usb_purge_tx_buffer(self):
       )
 
   async def poll_modem_status(self) -> int:
-    next_command = FTDICommand(**self.cr.next_command())
+    next_command = FTDICommand(**self.cr.next_command(self._device_id))
     if not (
       next_command.module == "ftdi"
       and next_command.device_id == self._device_id
@@ -272,7 +272,7 @@ async def poll_modem_status(self) -> int:
     return int(next_command.data)
 
   async def write(self, data: bytes):
-    next_command = FTDICommand(**self.cr.next_command())
+    next_command = FTDICommand(**self.cr.next_command(self._device_id))
     if not (
       next_command.module == "ftdi"
       and next_command.device_id == self._device_id
@@ -284,7 +284,7 @@ async def write(self, data: bytes):
       raise ValidationError("Data mismatch: difference was written to stdout.")
 
   async def read(self, num_bytes: int = 1) -> bytes:
-    next_command = FTDICommand(**self.cr.next_command())
+    next_command = FTDICommand(**self.cr.next_command(self._device_id))
     if not (
       next_command.module == "ftdi"
       and next_command.device_id == self._device_id
@@ -295,7 +295,7 @@ async def read(self, num_bytes: int = 1) -> bytes:
     return bytes.fromhex(next_command.data)
 
   async def readline(self) -> bytes:  # type: ignore # very dumb it's reading from pyserial
-    next_command = FTDICommand(**self.cr.next_command())
+    next_command = FTDICommand(**self.cr.next_command(self._device_id))
     if not (
       next_command.module == "ftdi"
       and next_command.device_id == self._device_id
diff --git a/pylabrobot/io/serial.py b/pylabrobot/io/serial.py
index b2706182cf..5609102ba5 100644
--- a/pylabrobot/io/serial.py
+++ b/pylabrobot/io/serial.py
@@ -220,7 +220,7 @@ async def setup(self):
     pass
 
   async def write(self, data: bytes):
-    next_command = SerialCommand(**self.cr.next_command())
+    next_command = SerialCommand(**self.cr.next_command(self._port))
     if not (
       next_command.module == "serial"
       and next_command.device_id == self._port
@@ -232,18 +232,18 @@ async def write(self, data: bytes):
       raise ValidationError("Data mismatch: difference was written to stdout.")
 
   async def read(self, num_bytes: int = 1) -> bytes:
-    next_command = SerialCommand(**self.cr.next_command())
+    next_command = SerialCommand(**self.cr.next_command(self._port))
     if not (
       next_command.module == "serial"
       and next_command.device_id == self._port
       and next_command.action == "read"
-      and len(next_command.data) == num_bytes
+      and len(next_command.data) <= num_bytes  # we can actually read less bytes than requested
     ):
       raise ValidationError(f"Next line is {next_command}, expected Serial read {num_bytes}")
     return next_command.data.encode()
 
   async def readline(self) -> bytes:  # type: ignore # very dumb it's reading from pyserial
-    next_command = SerialCommand(**self.cr.next_command())
+    next_command = SerialCommand(**self.cr.next_command(self._port))
     if not (
       next_command.module == "serial"
       and next_command.device_id == self._port
diff --git a/pylabrobot/io/usb.py b/pylabrobot/io/usb.py
index 768da645a5..91383033aa 100644
--- a/pylabrobot/io/usb.py
+++ b/pylabrobot/io/usb.py
@@ -403,7 +403,7 @@ async def setup(self):
     pass
 
   async def write(self, data: bytes, timeout: Optional[float] = None):
-    next_command = USBCommand(**self.cr.next_command())
+    next_command = USBCommand(**self.cr.next_command(self._unique_id))
     if not (
       next_command.module == "usb"
       and next_command.device_id == self._unique_id
@@ -415,7 +415,7 @@ async def write(self, data: bytes, timeout: Optional[float] = None):
       raise ValidationError("Data mismatch: difference was written to stdout.")
 
   async def read(self, timeout: Optional[float] = None) -> bytes:
-    next_command = USBCommand(**self.cr.next_command())
+    next_command = USBCommand(**self.cr.next_command(self._unique_id))
     if not (
       next_command.module == "usb"
       and next_command.device_id == self._unique_id