Revert Keithley22XX and Keithley24XX to use :READ? for measuring stuff.

Use :READ? instead of :MEAS:CURR? / :MEAS:VOLT? for measuring current/voltage of the Keithley power supplies. This reverts to the behaviour before the transition to IPowerSupply.

The difference between the two, according to the 2410 manual, is that the :MEAS? also turns-on the power supply if not already on. This seems a bit dangerous.

Also add a Keithley24XX::isOn function that returns true if the Keithley is turned on. It is used to protect :READ? statements, which hang if the power supply is off.

src/libPS/Keithley22XX.cpp

@@ -52,6 +52,13 @@ void Keithley22XX::turnOff(unsigned channel)
   m_com->send("CHAN:OUTP 0");
 }
 
+bool Keithley22XX::isOn(unsigned channel)
+{
+  if (channel != 1) throw std::runtime_error("Set the channel to 1 for single channel power-supply");
+
+  return m_com->sendreceive(":OUTPUT?")=="1";
+}
+
 void Keithley22XX::setCurrentLevel(double cur, unsigned channel)
 {
   if(channel!=1 && channel!=2 && channel!=3)
@@ -94,9 +101,13 @@ double Keithley22XX::measureCurrent(unsigned channel)
   if(channel!=1 && channel!=2 && channel!=3)
     throw std::runtime_error("Invalid channel: "+std::to_string(channel));
 
+  if(!isOn(channel))
+    return 0.;
+  
   m_com->send(":FORMAT:ELEMENTS CURR");
   m_com->send("INST CH" + std::to_string(channel));
-  return std::stod(m_com->sendreceive(":MEAS:CURR?"));
+  
+  return std::stod(m_com->sendreceive(":READ?"));
 }
 
 void Keithley22XX::setVoltageLevel(double volt, unsigned channel)
@@ -141,7 +152,12 @@ double Keithley22XX::measureVoltage(unsigned channel)
   if(channel!=1 && channel!=2 && channel!=3)
     throw std::runtime_error("Invalid channel: "+std::to_string(channel));
 
+  if(!isOn(channel))
+    return 0.;
+
   m_com->send(":FORMAT:ELEMENTS VOLT");
   m_com->send("INST CH" + std::to_string(channel));
-  return std::stod(m_com->sendreceive(":MEAS:VOLT?"));
+
+  return std::stod(m_com->sendreceive(":READ?"));
 }
+