Send test config (8C 2B) before single-shot measurement

The device needs the test config commands to enter single-test mode.
Without them it behaves as continuous start+stop. Supports auto_psu flag
so the device can handle PSU on/off automatically.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

hpcs6500.py

@@ -39,6 +39,19 @@ CMD_SET_AC = 0x78
 CMD_READ_PSU = 0x79
 CMD_PSU_OUTPUT = 0x72
 CMD_SET_MODE = 0x7A
+CMD_TEST_CONFIG = 0x2B
+
+# Single-test config payloads captured from vendor software.
+# Sub 00: test parameters, sub 01: test config with auto-PSU flag at byte 37.
+_TEST_CONFIG_SUB0 = bytes.fromhex(
+    "00003c00000001008813037c011a04000000484200000208"
+    "000000000001060000000000803f0000803f0000803f0000"
+    "803f0000803f00000000003f"
+)
+_TEST_CONFIG_SUB1_BASE = bytes.fromhex(
+    "017c010c037c010c037c010c030000803f000000000000000000000000"
+    "00000000000000000001000000400000803f"
+)
 
 # State polling byte[5] values
 STATE_IDLE = 0x04
@@ -268,6 +281,23 @@ class HPCS6500:
         state = resp[5]
         return data_available, state
 
+    def send_test_config(self, auto_psu=False):
+        """Send single-test configuration (8C 2B sub 00 + sub 01).
+        If auto_psu=True, device handles PSU on/off automatically."""
+        self.ser.reset_input_buffer()
+        self.ser.write(bytes([SYNC, CMD_TEST_CONFIG]) + _TEST_CONFIG_SUB0)
+        resp = self._recv(2, timeout=2.0)
+        if resp != bytes([SYNC, CMD_TEST_CONFIG]):
+            return False
+
+        sub1 = bytearray(_TEST_CONFIG_SUB1_BASE)
+        if auto_psu:
+            sub1[37] = 0x01
+        self.ser.reset_input_buffer()
+        self.ser.write(bytes([SYNC, CMD_TEST_CONFIG]) + bytes(sub1))
+        resp = self._recv(2, timeout=2.0)
+        return resp == bytes([SYNC, CMD_TEST_CONFIG])
+
     def start_measurement(self):
         """Send 8C 0E 01 to start measurement."""
         self.ser.reset_input_buffer()
@@ -426,10 +456,12 @@ class HPCS6500:
 
         return result
 
-    def take_single_reading(self):
-        """Trigger a single measurement (no PSU control).
-        Sends stop to trigger a one-shot reading, waits for data, reads it,
-        then resets. This matches the vendor software's single-test flow."""
+    def take_single_reading(self, auto_psu=False):
+        """Trigger a single measurement matching the vendor's single-test flow.
+        Sends test config, then stop to trigger a one-shot reading, waits for
+        data, reads it, then resets.
+        If auto_psu=True, the device handles PSU on/off automatically."""
+        self.send_test_config(auto_psu=auto_psu)
         self.stop_measurement()
 
         if not self.wait_for_data(timeout=30.0):