Port manager

SweepMe! supports and simplifies communication to devices via RS232/RS485 (COM port), GPIB, USB and TCPIP by using the SweepMe!'s Port manager.

Introduction

The communication with the hardware is usually done in each Device Class. In order to simplify the process of creating the communication channel, i.e. establish a connection, SweepMe! provides the port manager for easy handling.

Supported protocols

• COM: standard PC serial port using RS-232 or RS-485 protocol
• GPIB: a standardized communication/interface via an IEEE-488-Bus
• TCPIP: communication via a network, e.g. ethernet
• USB: plug and play USB test and measurement device as supported by pyvisa
• (VB: a National Instrument VirtualBench device -> deprecated from 1.5.5: please find and create port objects yourself)

Usage

Configuration

The following variables have to be set within the __init__ function of a Device Class:

In order to get the available ports listed in the GUI of the corresponding Module, choose desired port types by:

self.port_types = ["COM", "GPIB", "USB", "TCPIP"]

A port object is automatically created by

self.port_manager = True

Port properties

You can set port properties during __init__ of the Device Class or before connect() is called by modifying the self.port_properties dictionary. In order to see all keys of the dictionary, use

print(self.port_properties)

These parameters can be changed through

self.port_properties = {
    "baudrate" : 9600, 
    "EOL": "\n",
    ...
}

or change a parameter of self.port_properties as done in the the following example

self.port_properties["EOL"] = "\r"

Some important and commonly used parameters are explained here:

Command	Interface	Type	Default value	Explanation
EOL	COM	String	"\n"	"End of line"-character, will be added to each command sent by self.port.write() and defines the end of each message read by self.port.read()
EOLwrite	COM	String	None	replaces the character which will be added to each command sent by self.port.write()
EOLread	COM	String	None	replace the character which defines the end of each message read by self.port.read()
GPIB_EOLwrite	GPIB	String	None	sets the character which will be added to each command sent by self.port.write()
GPIB_EOLread	GPIB	String	None	sets the character which defines the end of each message read by self.port.read()
baudrate	COM	Integer	9600	The Baudrate defines the speed of the COM port communication, must be identical to the value of the instrument
bytesize	COM	Integer	8	length of one byte, must be identical to value of the instrument
stopbyte	COM	Integer	1
parity	COM	String	"N"
xonxoff	COM	bool	False
rtscts	COM	bool	False
dsrdtr	COM	bool	False
rts	COM	bool	True
dtr	COM	bool	True
raw_write	COM	bool	False	send the argument as is without further unicode encoding, an eol/terminator-character is still added if specified.
raw_read	COM	bool	False	receive the answer as is without further unicode decoding
timeout	COM, USB, GPIB	Float	1.0	time in seconds before a timeout error is raised
delay	COM, GPIB	Float	0.0	delay time in seconds after writing a command to allow an instrument to process the current command.
Exception	COM, GPIB, USB, TCPIP	bool	True	If an error occurs, the measurement is stopped. Set to False to be able to catch communications errors with try-except.

Communication

If the port manager is activated, the port is automatically available within all functions of the sequencer procedure as the variable:

self.port

A message can be sent by using:

self.port.write("string of the message without end-of-line/terminator character")

The end-of-line character ("EOL") will be set by changing the port properties

The answer of a device is acquired by:

var = self.port.read()

The returned object is typically a string, so please do not forget to change the type if needed by using e.g. int() or float().

In case you need direct access to the port object created by the port manager, use:

self.port.port

Thus, you can use commmands as defined by pyserial for COM port communication or pyvisa for GPIB/USB communication.

Terminator / End-of-Line character for GPIB

Typically, communication via GPIB does not require to set an end-of-line (EOL) character. However, some devices expect and send special characters at the end of each message indicating that the command/message is complete.

At the moment, setting EOL for GPIB is not implemented via self.port_properties. One has to modify the pyvisa GPIBInstrument object, using

self.port.port.write_termination = "\r\n"
self.port.port.read_termination = "\r\n"
# "\r\n" is here one example for using Carriage Return (CR) and Line Feed (LF) as terminator

which can be done e.g. in the connect() function of the DeviceClass

COM port

In order to check the number of already received bytes, you can use:

number_of_received_bytes = self.port.in_waiting()

As SweepMe! is based on python 3, all strings that are send via self.port.write() are actually unicode strings. The Port Manager automatically converts them to bytes before sending them using the library pyserial. Similarly, received bytes are automatically converted back to unicode strings. If you like to have raw access to the bytes that are sent and received, please use the port properties 'raw_read' and 'raw_write'.

Sometimes it is needed to only raw read only the answers of a few commands. In that case you can use the function 'read_raw' and 'write_raw'

self.port.write_raw(...) # make sure that you send a byte string
answer = self.port.read_raw() # answer will be a byte string that you need to decode or process yourself.
answer = self.port.read_raw(10) # example to read 10 bytes

Exceptions

If during port communication, an exception occurs, SweepMe! will stop the run and print the message in the Debug widget. If you like to catch the exception yourself, please add the key 'Exception' to the port properties and set it to 'False'.

self.port_properties = {
    ...   # all other properties
    "Exception": "False",
}

Then, you can use try-except to handle the exception, e.g. to send a command again if a checksum was incorrect or an instrument did not answer in the given timeout.

self.port.write("*IDN?")
try:
   self.port.read() # We try to read the answer
except:
    self.port.write("*IDN?") # If reading the answer fails, e.g. resulting in a timeout error, we write again to the instrument
    self.port.read()  # and we check the answer again. This example can be done multiple times in a loop if you like to implement multiple retries
}

pysweepme

The port manager of SweepMe! uses the module 'Ports' of the open-source pysweepme package. If you like to see how the exact communication is done, please feel free to download pysweepme and see the source code.