Added arbitrary waveform support. Arbitrary waveforms can be loaded f…

…rom a file that contains 2048 lines of samples in the range -1.0 to 1.0.

Added built-in sinc and normsinc waveforms using the arbitrary waveform machinery. They are generated on the fly and stored in slot 15.
Improved reliability of serial communications by implementing our own serial read loop that supports multiple reads and custom timeouts. This was needed because setting an arbitrary waveform slows down the instrument and it can take up to 300ms to respond.

README.md

@@ -39,6 +39,9 @@ options can be zero or more of the following:
     	verbose level
 
 command can be one or more of the following:
+
+  help - show command usage
+  
   showconfig - show the configuration of the current channel
   on - turn output on
   off - turn output off
@@ -60,6 +63,9 @@ command can be one or more of the following:
   sweepon - turn sweep function on
   sweepoff - turn sweep function off
 
+  slot N - set the arbitrary waveform slot to write to
+  arbwaveform file - set arbitrary waveform from file. The file should contain 2048 lines, 1 sample per line in the -1.0 to 1.0 range
+  
   measure cmd - measure values from waveform on ext-input. cmd can be one of frequency, count, period, pulsewidth, duty, negativepulsewidth, stop
 
   sleep N - delay N seconds before executing the next command
@@ -206,6 +212,8 @@ Additional Links
 
 Credit for all of the hard work goes to user `wd5gnr` from [EEV Blog](https://www.eevblog.com/) for reverse engineering the protcol and documenting it here: [MHS5200A Protocol](https://docs.google.com/document/d/1HbLQ4u87RJkD3Ktyw7k9U7Zh5BPNzbrhMlszNGdXiiY/edit)
 
+Credit for the sample arbitrary waveforms included in the waves directory got to user `wd5gnr` and can be found at his github repo here: [wd5gnr/mhs5200a](https://github.com/wd5gnr/mhs5200a)
+
 - [MHS-5200A Serial Protocol Reverse Engineered](https://www.eevblog.com/forum/testgear/mhs-5200a-serial-protocol-reverse-engineered/)
 
 - [MHS-5200A function generator teardown / review / reverse engineering](https://www.eevblog.com/forum/testgear/mhs-5200a-function-generator-teardown-review-reverse-engineering/)