Faster qdac (#53)

* add support for faster setting of qdac voltage

* SR830 fail early in buffered readout if butffer is not full as expected

* add option to qdac to ignore return read on voltage set

This ivery experimental and likely to break if you are not careful

qcodes/instrument_drivers/QDev/QDac.py

@@ -156,6 +156,22 @@ def __init__(self, name, address, num_chans=48, update_currents=True):
                            set_cmd='ver {}',
                            val_mapping={True: 1, False: 0})
 
+        self.add_parameter(name='fast_voltage_set',
+                           label='fast voltage set',
+                           parameter_class=ManualParameter,
+                           vals=vals.Bool(),
+                           initial_value=False,
+                           docstring=""""Toggles if DC voltage set should unset any ramp attached to this channel.
+                                     If you enable this you should ensure thay any function generator is unset
+                                     from the channel before setting voltage""")
+        self.add_parameter(name='voltage_set_dont_wait',
+                           label='voltage set dont wait',
+                           parameter_class=ManualParameter,
+                           vals=vals.Bool(),
+                           initial_value=False,
+                           docstring=""""If enabled the voltage set does not wait for a reply from the 
+                           qdac. This is experimental and may break but has the potential to speed up
+                           stepping the qdac""")
         # Initialise the instrument, all channels DC (unbind func. generators)
         for chan in self.chan_range:
             # Note: this call does NOT change the voltage on the channel
@@ -208,18 +224,19 @@ def _set_voltage(self, chan, v_set):
             self._assigned_fgs[chan] = fg
             # We need .get and not get_latest in case a ramp was interrupted
             v_start = self.parameters['ch{:02}_v'.format(chan)].get()
-            time = abs(v_set-v_start)/slope
-            log.info('Slope: {}, time: {}'.format(slope, time))
+            mytime = abs(v_set-v_start)/slope
+            log.info('Slope: {}, time: {}'.format(slope, mytime))
             # Attenuation compensation and syncing
             # happen inside _rampvoltage
-            self._rampvoltage(chan, fg, v_start, v_set, time)
+            self._rampvoltage(chan, fg, v_start, v_set, mytime)
         else:
             # compensate for the 0.1 multiplier, if it's on
             if self.parameters['ch{:02}_vrange'.format(chan)].get_latest() == 1:
                 v_set = v_set*10
             # set the mode back to DC in case it had been changed
-            self.write('wav {} 0 0 0'.format(chan))
-            self.write('set {} {:.6f}'.format(chan, v_set))
+            if not self.fast_voltage_set():
+                self.write('wav {} 0 0 0'.format(chan))
+            self.write('set {} {:.6f}'.format(chan, v_set), fast=self.voltage_set_dont_wait())
 
     def _set_vrange(self, chan, switchint):
         """
@@ -535,7 +552,7 @@ def _rampvoltage(self, chan, fg, v_start, setvoltage, ramptime):
         self.write(funmssg)
 
 
-    def write(self, cmd):
+    def write(self, cmd, fast=False):
         """
         QDac always returns something even from set commands, even when
         verbose mode is off, so we'll override write to take this out
@@ -553,7 +570,8 @@ def write(self, cmd):
 
         nr_bytes_written, ret_code = self.visa_handle.write(cmd)
         self.check_error(ret_code)
-        self._write_response = self.visa_handle.read()
+        if not fast:
+            self._write_response = self.visa_handle.read()
 
     def read(self):
         return self.visa_handle.read()

qcodes/instrument_drivers/stanford_research/SR830.py

@@ -108,7 +108,8 @@ def get(self):
         # parse it
         realdata = np.fromstring(rawdata, dtype='<i2')
         numbers = realdata[::2]*2.0**(realdata[1::2]-124)
-
+        if self.shape[0] != N:
+            raise RuntimeError("SR830 got {} points in buffer expected {}".format(N, self.shape[0]))
         return numbers