Fixed bug in USBTMC code which caused issues when packets were longer than max usb packet size

pyvisa_py/protocols/usbtmc.py

@@ -479,13 +479,19 @@
             try:
                 resp = raw_read(recv_chunk + header_size + max_padding)
                 response = BulkInMessage.from_bytes(resp)
+                received.extend(response.data)
+                while len(resp) == self.usb_recv_ep.wMaxPacketSize:
+                    # USBTMC Section 3.3 specifies that the first usb packet
+                    # must contain the header. the remaining packets do not need
+                    # the header the message is finished when a "short packet"
+                    # is sent (one whose length is less than wMaxPacketSize)
+                    resp = raw_read(recv_chunk + header_size + max_padding)
+                    received.extend(resp)
             except (usb.core.USBError, ValueError):
                 # Abort failed Bulk-IN operation.
                 self._abort_bulk_in(self._btag)
                 raise
 
-            received.extend(response.data)
-
             # Detect EOM only when device sends all expected bytes.
             if len(response.data) >= response.transfer_size:
                 eom = response.transfer_attributes & 1

pyvisa_py/usb.py

@@ -306,6 +306,55 @@
             )
         return out
 
+    def read(self, count: int) -> Tuple[bytes, StatusCode]:
+        """Reads data from device or interface synchronously.
+
+        Corresponds to viRead function of the VISA library.
+
+        Parameters
+        -----------
+        count : int
+            Number of bytes to be read.
+
+        Returns
+        -------
+        bytes
+            Data read from the device
+        StatusCode
+            Return value of the library call.
+
+        """
+
+        def _usb_reader():
+            """Data reader identifying usb timeout exception."""
+            try:
+                return self.interface.read(count)
+            except usb.USBError as exc:
+                if exc.errno in (errno.ETIMEDOUT, -errno.ETIMEDOUT):
+                    raise USBTimeoutException()
+                raise
+
+        supress_end_en, _ = self.get_attribute(ResourceAttribute.suppress_end_enabled)
+
+        if supress_end_en:
+            raise ValueError(
+                "VI_ATTR_SUPPRESS_END_EN == True is currently unsupported by pyvisa-py"
+            )
+
+        term_char, _ = self.get_attribute(ResourceAttribute.termchar)
+        term_char_en, _ = self.get_attribute(ResourceAttribute.termchar_enabled)
+
+        return self._read(
+            _usb_reader,
+            count,
+            lambda current: False,  # USBTMC can return partial message (i.e.,
+            # before the term_char) or have trailing zeros
+            supress_end_en,
+            term_char,
+            term_char_en,
+            USBTimeoutException,
+        )
+
 
 @Session.register(constants.InterfaceType.usb, "RAW")
 class USBRawSession(USBSession):