Python script to read data from VA18b multimeter.

Some years ago I wanted a cheap multimeter with pc interface. The one I was able to obtain quickly was the VA18b. There was no official software for GNU/Linux. There is a Perl script and some C implementations. I wanted to interface multiple serial devices (temperature sensor + the DMM) so I wrote a Python script to do so. I will describe the part for the multimeter. The nice thing is that the communication protocol is known. The adapter is just a generic USBSerial (Prolific pl2303 or Sunplus SPCP8x5) device. I creates a ttyUSBx device.

The way those cheap multimeter usually work is by sending the state of every segment of the LCD. My VA18b has an LCD (image bellow) that consists of 56 segments. So it need to send 56 bit to describe the sate of every one. The multimeter actually sends 14 bytes (14×8=112 bits) but uses only the low nibble of the byte to transfer the segment state. The serial port setting is 2400 baud, 8 bits, Stop 1, Parity none.

#!/usr/bin/python2.7

#
#	Just a quick n' dirty script to read the Data from a VA18B multimeter.
#	This DMM uses the low nibble of 14 bytes to send the lcd segments.
#
#       License: GPL v3. http://www.gnu.org/licenses/gpl-3.0-standalone.html
#

import signal
import sys
import time
import serial
import io
import getopt

interval = '1.0'
device = '/dev/ttyUSB0' 


def showhelp():
	print '        va18b.py -i <time in secs> -d <device>'
	print '   Eg:  va18b.py -i 1.5 -d /dev/ttyUSB1'
	
	
try:
	opts, args = getopt.getopt(sys.argv[1:],"hd:i:",['help'])
except getopt.GetoptError:
	print 'Error: -i, -d option require a value.'
	showhelp()
	sys.exit(1)
        
for opt, arg in opts:
	if opt in ('-h','--help'):
	  showhelp()
	  sys.exit(1)
	if opt == '-i':
	  interval = arg
	if opt == '-d':
	  device = arg
	  
try:
	port=serial.Serial(port=device,
			   baudrate=2400,
			   bytesize=serial.EIGHTBITS,
			   stopbits=serial.STOPBITS_ONE,
			   parity=serial.PARITY_NONE,
			   timeout=None)
	
	if not port.isOpen():
		port.open()

except IOError,err:
	print '\nError:' + str(err) + '\n'
	sys.exit(1)


def signal_handler(signal, frame):
        sys.stderr.write('\n\nYou pressed Ctrl+C!\n\n')
        port.flushInput()
	port.close()
        sys.exit(0)
        
signal.signal(signal.SIGINT, signal_handler)

# Every packet is 14 bytes long.
def get_bytes():
	i=0
	substr=''
	while i<=13:
		byte=port.read(1)
		# converting every byte to binary format keeping the low nibble.
		substr += '{0:08b}'.format(ord(byte))[4:]
		i += 1
	return substr
      

def stream_decode(substr):
  
	ac       = int(substr[0:1])
	dc       = int(substr[1:2])
	auto     = int(substr[2:3])
	pclink   = substr[3:4]
	minus    = int(substr[4:5])
	
	digit1   = substr[5:12]
	dot1     = int(substr[12:13])
	digit2   = substr[13:20]
	dot2     = int(substr[20:21])
	digit3   = substr[21:28]
	dot3     = int(substr[28:29])
	digit4   = substr[29:36]
	
	micro    = int(substr[36:37])
	nano     = int(substr[37:38])
	kilo     = int(substr[38:39])
	diotst   = int(substr[39:40])
	mili     = int(substr[40:41])
	percent  = int(substr[41:42])
	mega     = int(substr[42:43])
	contst   = int(substr[43:44])
	cap      = int(substr[44:45])
	ohm      = int(substr[45:46])
	rel      = int(substr[46:47])
	hold     = int(substr[47:48])
	amp      = int(substr[48:49])
	volts    = int(substr[49:50])
	hertz    = int(substr[50:51])
	lowbat   = int(substr[51:52])
	minm     = int(substr[52:53])
	fahrenh  = substr[53:54]
	celcius  = int(substr[54:55])
	maxm     = int(substr[55:56])
	
	digit = {"1111101":"0",
	         "0000101":"1",
	         "1011011":"2",
	         "0011111":"3",
	         "0100111":"4",
	         "0111110":"5",
	         "1111110":"6",
	         "0010101":"7",
	         "1111111":"8",
	         "0111111":"9",
	         "0000000":"",
	         "1101000":"L"}
	
	value = ("-" if minus else " ") +\
	        digit.get(digit1,"") + ("." if dot1 else "") +\
	        digit.get(digit2,"") + ("." if dot2 else "") +\
	        digit.get(digit3,"") + ("." if dot3 else "") +\
	        digit.get(digit4,"")

	flags = " ".join(["AC"         if ac     else "",
	                  "DC"         if dc     else "",
	                  "Auto"       if auto   else "",
	                  "Diode test" if diotst else "",
	                  "Conti test" if contst else "",
	                  "Capacity"   if cap    else "",
	                  "Rel"        if rel    else "",
	                  "Hold"       if hold   else "",
	                  "Min"        if minm   else "",
	                  "Max"        if maxm   else "",
	                  "LowBat"     if lowbat else ""])
	                 
	units = ("n"    if nano    else "") +\
	        ("u"    if micro   else "") +\
	        ("k"    if kilo    else "") +\
	        ("m"    if mili    else "") +\
	        ("M"    if mega    else "") +\
	        ("%"    if percent else "") +\
	        ("Ohm"  if ohm     else "") +\
	        ("Amp"  if amp     else "") +\
	        ("Volt" if volts   else "") +\
	        ("Hz"   if hertz   else "") +\
	        ("C"    if celcius else "")
	       
	return value + " " + flags + " " + units
	
	
sys.stderr.write('Press Ctrl+C to exit.\n\n')

while 1:
	substr = get_bytes()
	data = stream_decode(substr)

	print data
	time.sleep(float(interval))
	port.flushInput()