A resource represents an instrument, e.g. a measurement device. There are multiple classes derived from resources representing the different available types of resources (eg. GPIB, Serial). Each contains the particular set of attributes an methods that are available by the underlying device.
You do not create this objects directly but they are returned by the open_resource method of a ResourceManager. In general terms, there are two main groups derived from Resource: MessageBased and RegisterBased.
The following sections explore the most common attributes of Resource and MessageBased (Serial, GPIB, etc) which are the ones you will encounte more often. For more information, refer to the API.
Each communication channel to an instrument has a session handle which is unique. You can get this value:
>>> my_device.session 10442240
If the resource is closed, an exception will be raised:
>>> inst.close() >>> inst.session Traceback (most recent call last): ... pyvisa.errors.InvalidSession: Invalid session handle. The resource might be closed.
Very most VISA I/O operations may be performed with a timeout. If a timeout is set, every operation that takes longer than the timeout is aborted and an exception is raised. Timeouts are given per instrument in milliseconds.
For all PyVISA objects, a timeout is set with
my_device.timeout = 25000
Here, my_device may be a device, an interface or whatever, and its timeout is set to 25 seconds. To set an infinite timeout, set it to None or float(‘+inf’):
To set it to immediate, set it to 0 or a negative value.
Now every operation of the resource takes as long as it takes, even indefinitely if necessary.
Attributes of MessageBase resources¶
If you read data from a device, you must store it somewhere. Unfortunately, PyVISA must make space for the data before it starts reading, which means that it must know how much data the device will send. However, it doesn’t know a priori.
Therefore, PyVISA reads from the device in chunks. Each chunk is 20 kilobytes long by default. If there’s still data to be read, PyVISA repeats the procedure and eventually concatenates the results and returns it to you. Those 20 kilobytes are large enough so that mostly one read cycle is sufficient.
The whole thing happens automatically, as you can see. Normally you needn’t worry about it. However, some devices don’t like to send data in chunks. So if you have trouble with a certain device and expect data lengths larger than the default chunk length, you should increase its value by saying e.g.
my_instrument.chunk_size = 102400
This example sets it to 100 kilobytes.
Somehow the computer must detect when the device is finished with sending a message. It does so by using different methods, depending on the bus system. In most cases you don’t need to worry about termination characters because the defaults are very good. However, if you have trouble, you may influence termination characters with PyVISA.
Termination characters may be one character or a sequence of characters. Whenever this character or sequence occurs in the input stream, the read operation is terminated and the read message is given to the calling application. The next read operation continues with the input stream immediately after the last termination sequence. In PyVISA, the termination characters are stripped off the message before it is given to you.
You may set termination characters for each instrument, e.g.
my_instrument.read_termination = '\r'
(‘r’ is carriage return, usually appearing in the manuals as CR)
Alternatively you can give it when creating your instrument object:
my_instrument = rm.open_resource("GPIB::10", read_termination='\r')
The default value depends on the bus system. Generally, the sequence is empty, in particular for GPIB. For RS232 it’s r.
You can specify the character to add to each outgoing message using the write_termination attribute.
query_delay and send_end¶
There are two further options related to message termination, namely send_end and query_delay. send_end is a boolean. If it’s True (the default), the EOI line is asserted after each write operation, signalling the end of the operation. EOI is GPIB-specific but similar action is taken for other interfaces.
The argument query_delay is the time in seconds to wait after each write operation. So you could write:
my_instrument = rm.open_resource("GPIB::10", send_end=False, delay=1.2)
This will set the delay to 1.2 seconds, and the EOI line is omitted. By the way, omitting EOI is not recommended, so if you omit it nevertheless, you should know what you’re doing.