B&K Precision 8500-8526 - Manual - Manual (Page 49)

if i % 10 == 0 and i != 0:
print
print header,
if i % 5 == 0:
print " ",
s = "%02x" % ord(bytes[i])
if s == "00":
s = chr(250)*2
print s,
print
def CalculateChecksum(cmd):
assert((len(cmd) == length_packet - 1) or (len(cmd) ==
length_packet))
checksum = 0
for i in xrange(length_packet - 1):
checksum += ord(cmd[i])
checksum %= 256
return checksum
def main():
port = 3 # COM4 for my computer
baudrate = 38400
sp = serial.Serial(port, baudrate) # Open a serial connection
# Construct a set to remote command
cmd = chr(0xaa) + chr(0x00) + chr(0x20) # First three bytes
cmd += chr(0x01) + chr(0x00)*(length_packet - 1 - 4)
cmd += chr(CalculateChecksum(cmd))
assert(len(cmd) == length_packet)
# Send command to DC load
sp.write(cmd)
print "Set to remote command:"
DumpCommand(cmd)
# Get response from DC load
response = sp.read(length_packet)
assert(len(response) == length_packet)
print "Response:"
DumpCommand(response)
main()
The first three lines of the main() function set up a serial port to talk to. The next five lines construct
the string that we will send to the DC load. The chr() function creates a single character that has
the ASCII value of the argument. The + symbols allow strings to be concatenated. The expression
chr(0)*a_number creates a string of ASCII 0x00 characters whose length is a_number . . The last
character is the checksum of the previous 25 characters, calculated for us by the
CalculateChecksum() function.
When a command is sent to the instrument, you must always request for return data, which will
always be another 26 bytes. This is also dumped to the screen.
8500 DC Load Series Version: 030614 Page 49 of 77