meerstetterTEC: Fix parsing of error replies

meerstetterTEC.py

@@ -14,6 +14,7 @@ from ptblab import datalogger, terminal, utils
 EOF = b'\r'
 DEVICE = b'!'
 HOST = b'#'
+ERROR = b'+'
 
 
 class Errno(int, enum.Enum):
@@ -102,19 +103,27 @@ class MeCom:
 
     def recv(self, crc=None):
         packet = self.conn.read_until(EOF).rstrip()
-        crc = crc if crc is not None else self.crc(packet[:-4])
+        control, address, seqno, data = self.unpack(packet[:-4])
+
+        # When the received frame is an ACK to a VS instruction, the
+        # CRC matches the CRC of the packet containing the VS
+        # instruction.  When the received frame is an error or a reply
+        # to other instructions, the CRC should match the CRC of the
+        # reply packet. When the reply is expected in response to a VS
+        # instruction, the expected CRC is provided as an argument.
+        crc = crc if crc is not None and data[0:1] != ERROR else self.crc(packet[:-4])
+
         if not packet:
             raise TimeoutError
         if crc != packet[-4:]:
             raise ValueError(f'CRC error: {packet}')
-        control, address, seqno, data = self.unpack(packet[:-4])
         if control != DEVICE:
             raise ValueError(f'unexpected control character: {packet}')
         if address != self.address:
             raise ValueError(f'unexpected return address: {packet}')
         if seqno != self.seqno:
             raise ValueError(f'unexpected return seqno: {packet}')
-        if data[0:1] == b'+':
+        if data[0:1] == ERROR:
             raise MeComError(Errno(int(data[1:])))
         return data