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

CHANGES

@@ -5,6 +5,10 @@ PyVISA-py Changelog
 ------------------
 
 - add read_stb method for TCPIP HiSLIP client PR #429
+- fix usbtmc implementation to respect section 3.3 of the spec PR #449
+  Read now reads from usb until a "short packet" has been read
+  (see specification), and only expects a header on the first packet received.
+- add support for VI_ATTR_SUPPRESS_END_EN for USB resources PR #449
 
 0.7.2 (07/03/2024)
 ------------------

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/sessions.py

@@ -796,10 +796,9 @@
             if current:
                 out.extend(current)
                 end_indicator_received = end_indicator_checker(current)
-                if end_indicator_received:
-                    if not suppress_end_en:
-                        # RULE 6.1.1
-                        return bytes(out), StatusCode.success
+                if end_indicator_received and not suppress_end_en:
+                    # RULE 6.1.1
+                    return bytes(out), StatusCode.success
                 else:
                     if termination_char_en and (term_char in current):
                         # RULE 6.1.2

pyvisa_py/usb.py

@@ -157,11 +157,6 @@ def _usb_reader():
 
         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)