Bk Precision BK-8500 Electronic Load 1 mA - 30 A 0.1 - 120 Vdc 0 - 300 W BK-8500 Datenbogen

        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 

Calcu la t eChecksum( )

 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.