qmi.instruments.newport.newport_843r_libusb

Basic input/output access to the Newport 843-R power meter via USB.

This Python 3 module provides access to ASCII command interface of the Newport 843-R-USB via its proprietary USB protocol.

This module uses PyUSB and works only on Linux.

Access permission

To access the instrument, read/write permission to the USB device node (/dev/usb/<bus>/<addr>) is required. This is typically accomplished on Linux by adding a suitable rule in /etc/udev/rules.d. For example:

/etc/udev/rules.d/99-newport_usb.rules: SUBYSTEM==”usb”, ATTRS{idVendor}==”0bd3”, ATTRS{idProduct}==”e345”, GROUP=”dialout”, MODE=”0660”

Command language

The USB access provided by this module enables the use of the ASCII command set of the 843-R instrument as documented by Newport. See “PMManager-User-Commands.pdf” or “Newport User Commands.pdf”.

Command strings start with “$”, followed by a two-letter command, optionally followed by parameters, and ending with ” “.

In case of success, the instrument sends a response string starting with “*”, followed by a command-specific response format, and ending with ” “.

If an error occurs, the instrument sends a response string starting with “?”, followed by an error message and ending with ” “.

Streaming mode

In addition to the command/response flow, the instrument also supports a streaming mode in which it spontaneously sends several power readings per second via the USB interrupt endpoint 0x82. A sequence of undocumented commands is required to enable this streaming mode. The streaming mode is currently not supported by this Python module.

Protocol notes

The instrument has USB vendor ID 0x0BD3, vendor string “Freescale”, product ID 0xE345, product string “843-R”.

A serial number string is present in the USB descriptor.

The USB device class is 255 (vendor-specified), subclass 0. The device supports 1 USB configuration, containing 1 interface. The interface class is 255 (vendor-specified), subclass 255.

The interface contains 4 USB endpoints:

EP 0x81: interrupt IN, max 512 bytes (purpose unknown)
EP 0x82: interrupt IN, max 512 bytes (used in streaming mode)
EP 0x83: bulk IN, max 512 bytes (purpose unknown)
EP 0x04: bulk OUT, max 512 bytes (purpose unknown)

The request/response flow based on ASCII strings is implemented via control transfers on the default control endpoint (EP 0x00).

To send a command string to the instrument, the computer initiates the following control transfer:

bmRequestType = 0x40 (host-to-device, vendor-specified, device request) bRequest = 2 wValue = 0 wIndex = 0 data = command string (starting with “$”, ending with “rn”)

To read a response string from the instrument, the computer initiates the following control transfer:

bmRequestType = 0xc0 (device-to-host, vendor-specifid, device request) bRequest = 4 wValue = 0 wIndex = 0 wLength = 2000

The device then responds with an answer string (starting with “*” or “?” and ending with ” “).

Write command and read response transactions are sent in strict alternation. Each command triggers exactly 1 response from the device.

The device-side implementation of this protocol is quite fragile. Attempting to read a response when the instrument is not ready to send a response will easily crash the USB interface to the point where it can no longer respond to USB descriptor requests.

Classes

Newport_843R_libusb(serial_number)

Basic input/output access to the Newport 843-R power meter via USB.

class qmi.instruments.newport.newport_843r_libusb.Newport_843R_libusb(serial_number: str | None)

Basic input/output access to the Newport 843-R power meter via USB.

classmethod list_instruments() → List[str]

Return serial numbers of attached Newport 843-R instruments.

Scan the USB bus for attached Newport 843-R instruments and return a list of their serial numbers.

close() → None

Release USB device.

Raises:: USBError – If an error occurs at the USB level.

send_command(cmd: bytes, timeout: float = 1.0) → None

Send a command to the instrument and return the response.

See the Newport documentation for a list of supported commands (PMManager-User-Commands.pdf).

Parameters:: cmd – Command string to send to the instrument. The command is typically an ASCII string, starting with “$” and ending with “rn”.
Returns:: Response from the instrument.
Raises:: USBError – If an error occurs on the USB level.

read_response(timeout: float = 1.0) → bytes

Read a response string from the instrument.

Warning: The USB interface in the instrument is extremely fickle. Attempting to read a response when the instrument is not ready to send a response, may crash the USB interface and require power-cycling of the instrument to recover.

Parameters:: timeout – Maximum time to wait for the response (seconds).
Returns:: Response from the instrument. The response is typically an ASCII string ending with “n”.
Raises:: USBError – If the device does not respond or when an error occurs on the USB level.