class Instrument(resource_name[, **keyw])

represents an instrument, e.g. a measurement device. It is independent of a particular bus system, i.e. it may be a GPIB, serial, USB, or whatever instrument. However, it is not possible to perform bus-specific operations on instruments created by this class. For this, have a look at the specialised classes like GpibInstrument (section Common properties of instrument variables).

The parameter resource_name takes the same syntax as resource specifiers in VISA. Thus, it begins with the bus system followed by ”::”, continues with the location of the device within the bus system, and ends with an optional ”::INSTR”.

Possible keyword arguments are:

Keyword Description
timeout timeout in seconds for all device operations, see section Timeouts. Default: 5
chunk_size Length of read data chunks in bytes, see section Chunk length. Default: 20kB
values_format Data format for lists of read values, see section Reading binary data. Default: ascii
term_char termination characters, see section Termination characters. Default: None
send_end whether to assert END after each write operation, see section Termination characters. Default: True
delay delay in seconds after each write operation, see section Termination characters. Default: 0
lock whether you want to have exclusive access to the device. Default: VI_NO_LOCK

For further information about the locking mechanism, see The VISA library implementation.

The class Instrument defines the following methods and attributes:


writes the string message to the instrument.

returns a string sent from the instrument to the computer.


returns a list of decimal values (floats) sent from the instrument to the computer. See section A more complex example above. The list may contain only one element or may be empty.

The optional format argument overrides the setting of values_format. For information about that, see section Reading binary data.


sends the string message to the instrument and returns the answer string from the instrument.

Instrument.ask_for_values(message[, format])

sends the string message to the instrument and reads the answer as a list of values, just as read_values() does.

The optional format argument overrides the setting of values_format. For information about that, see section Reading binary data.


resets the device. This operation is highly bus-dependent. I refer you to the original VISA documentation, which explains how this is achieved for VXI, GPIB, serial, etc.


sends a trigger signal to the instrument.


returns a string sent from the instrument to the computer. In contrast to read(), no termination characters are checked or stripped. You get the pristine message.


The timeout in seconds for each I/O operation. See section Timeouts for further information.


The termination characters for each read and write operation. See section Termination characters for further information.


Whether or not to assert EOI (or something equivalent, depending on the interface type) after each write operation. See section Termination characters for further information.


Time in seconds to wait after each write operation. See section Termination characters for further information.


The format for multi-value data sent from the instrument to the computer. See section Reading binary data for further information.

Common properties of instrument variables


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.

For all PyVISA objects, a timeout is set with

my_device.timeout = 25

Here, my_device may be a device, an interface or whatever, and its timeout is set to 25 seconds. Floating-point values are allowed. If you set it to zero, all operations must succeed instantaneously. You must not set it to None. Instead, if you want to remove the timeout, just say

del my_device.timeout

Now every operation of the resource takes as long as it takes, even indefinitely if necessary.

The default timeout is 5 seconds, but you can change it when creating the device object:

my_instrument = instrument("ASRL1", timeout = 8)

This creates the object variable my_instrument and sets its timeout to 8 seconds. In this context, a timeout value of None is allowed, which removes the timeout for this device.

Note that your local VISA library may round up this value heavily. I experienced this effect with my National Instruments VISA implementation, which rounds off to 0, 1, 3 and 10 seconds.

Chunk length

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.

Reading binary data

Some instruments allow for sending the measured data in binary form. This has the advantage that the data transfer is much smaller and takes less time. PyVISA currently supports three forms of transfers:

This is the default mode. It assumes a normal string with comma- or whitespace-separated values.
The values are expected as a binary sequence of IEEE floating point values with single precision (i.e. four bytes each). All flavours of binary data streams defined in IEEE488.2 are supported, i.e. those beginning with <header>#<digit>, where <header> is optional, and <digit> may also be “0”.
The same as single, but with values of double precision (eight bytes each).

You can set the form of transfer with the property values_format, either with the generation of the object,

from pyvisa.highlevel import ascii, single, double

my_instrument = instrument("GPIB::12", values_format = single)

or later by setting the property directly:

my_instrument.values_format = single

Setting this option affects the methods read_values() and ask_for_values(). In particular, you must assure separately that the device actually sends in this format. In some cases it may be necessary to set the byte order, also known as endianness. PyVISA assumes little-endian as default. Some instruments call this “swapped” byte order. However, there is also big-endian byte order. In this case you have to append | big_endian to your values format:

my_instrument = instrument("GPIB::12", values_format = single | big_endian)


In order to demonstrate how easy reading binary data can be, remember our example from section A more complex example. You just have to append the lines

keithley.write("format:data sreal")
keithley.values_format = single

to the initialisation commands, and all measurement data will be transmitted as binary. You will only notice the increased speed, as PyVISA converts it into the same list of values as before.

Termination characters

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.term_chars = CR

Alternatively you can give it when creating your instrument object:

my_instrument = instrument("GPIB::10", term_chars = CR)

The default value depends on the bus system. Generally, the sequence is empty, in particular for GPIB . For RS232 it’s CR .

Well, the real default is not “” (the empty string) but None. There is a subtle difference: “” really means the termination characters are not used at all, neither for read nor for write operations. In contrast, None means that every write operation is implicitly terminated with CR+LF . This works well with most instruments.

All CRs and LFs are stripped from the end of a read string, no matter how term_chars is set.

The termination characters sequence is an ordinary string. CR and LF are just string constants that allow readable access to “\r” and “\n”. Therefore, instead of CR+LF, you can also write “\r\n”, whichever you like more.

delay and send_end

There are two further options related to message termination, namely send_end and 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 delay is the time in seconds to wait after each write operation. So you could write:

my_instrument = instrument("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.

GPIB devices

class GpibInstrument(gpib_identifier[, board_number[, **keyw]])

represents a GPIB instrument. If gpib_identifier is a string, it is interpreted as a VISA resource name (section VISA resource names). If it is a number, it denotes the device number at the GPIB bus.

The optional board_number defaults to zero. If you have more that one GPIB bus system attached to the computer, you can select the bus with this parameter.

The keyword arguments are interpreted the same as with the class Instrument.


Since this class is derived from the class Instrument, please refer to section General devices for the basic operations. GpibInstrument can do everything that Instrument can do, so it simply extends the original class with GPIB-specific operations.

The class GpibInstrument defines the following methods:


waits for a serial request (SRQ) coming from the instrument. Note that this method is not ended when another instrument signals an SRQ, only this instrument.

The timeout argument, given in seconds, denotes the maximal waiting time. The default value is 25 (seconds). If you pass None for the timeout, this method waits forever if no SRQ arrives.

class Gpib([board_number])

represents a GPIB board. Although most setups have at most one GPIB interface card or USB-GPIB device (with board number 0), theoretically you may have more. Be that as it may, for board-level operations, i.e. operations that affect the whole bus with all connected devices, you must create an instance of this class.

The optional GPIB board number board_number defaults to 0.

The class Gpib defines the following method:


pulses the interface clear line (IFC) for at least 0.1 seconds.


You needn’t store the board instance in a variable. Instead, you may send an IFC signal just by saying Gpib().send_ifc().

Serial devices

Please note that “serial instrument” means only RS232 and parallel port instruments, i.e. everything attached to COM and LPT. In particular, it does not include USB instruments. For USB you have to use Instrument instead.

class SerialInstrument(resource_name[, **keyw])

represents a serial instrument. resource_name is the VISA resource name, see section VISA resource names. The general keyword arguments are interpreted the same as with the class Instrument. The only difference is the default value for term_chars: For serial instruments, CR (carriage return) is used to terminate readings and writings.


Since this class is derived from the class Instrument, please refer to section General devices for all operations. SerialInstrument can do everything that Instrument can do.

The class SerialInstrument defines the following additional properties. Note that all properties can also be given as keyword arguments when calling the class constructor or instrument().


The communication speed in baud. The default value is 9600.


Number of data bits contained in each frame. Its value must be from 5 to 8. The default is 8.


Number of stop bits contained in each frame. Possible values are 1, 1.5, and 2. The default is 1.


The parity used with every frame transmitted and received. Possible values are:

Value Description
no_parity no parity bit is used
odd_parity the parity bit causes odd parity
even_parity the parity bit causes even parity
mark_parity the parity bit exists but it’s always 1
space_parity the parity bit exists but it’s always 0

The default value is no_parity.


This determines the method used to terminate read operations. Possible values are:

Value Description
last_bit_end_input read will terminate as soon as a character arrives with its last data bit set
term_chars_end_input read will terminate as soon as the last character of term_chars is received

The default value is term_chars_end_input.