Reading The Gpib Address; Changing The Gpib Address; Basic Usb Concepts - Newport LDC-3706 Series User Manual

Reading the GPIB Address

Before operating the LDC-3706 remotely, its GPIB address must be known. Simply press the
LOCAL button in the COMM section of the front panel until GPIB Address is displayed on the
display. The factory default address is 1.

Changing the GPIB Address

Every device on the GPIB bus must have a unique address. If it is necessary to change the address,
press the LOCAL button in the COMM section of the front panel until the GPIB address is displayed.
Then adjust the knob until the desired address value is shown. The new GPIB address is stored in
nonvolatile memory when the display times out. The allowable address range is 1 – 30 for primary
GPIB addressing.

Basic USB Concepts

Universal Serial Bus (USB) is a specification to establish communication between devices and a
host controller, which has effectively replaced a variety of earlier interfaces such as serial and
parallel ports. There are several USB standards available, including USB 1.1, USB 2.0, and
USB 3.0. The LDC-3706 is designed to conform to the USB 2.0 standard and USBTMC 488
substandard. USB cables use 4 lines - Power, Ground and a twisted pair differential +/- data
lines. The USB connectors are designed so that power and ground are applied before the signal
lines are connected. When the host powers up it performs the enumeration process by polling
each device in turn, assigning each one a unique address and finding out from each device
what its speed is and what type of data transfer it wishes to perform. The enumeration process
also takes place whenever a device is plugged into an active network. The design of the
connectors, the process of enumeration, the host software, and the device firmware allows
devices to be described as "Plug-and-Play".
When the USB device is enumerated and gets an address from the host, it presents the host
with information about itself in the form of a series of descriptors. The device descriptor tells the
host the vendor and the product ID. The configuration descriptors offer a power consumption
value and a number of interface descriptors. Each of these interface descriptors define a
number of endpoints, which are the sources and destinations for data transfers. The endpoint
descriptors provide the following details: transfer type: bulk, interrupt or isochronous; direction;
packet sizes; bandwidth requirement; and repeat interval.
In USB communication, a typical transaction consists of a number of packets; a packet contains
a token indicating the type of data that the host is sending or requiring, the data, and in some
cases an acknowledgement. Each packet is preceded by a sync field and followed by an end of
packet marker. These transactions are used to provide four basic data transfer mechanisms,
including control, interrupt, bulk, and isochronous types.
USBTMC stands for USB Test and Measurement Class. USBTMC is a protocol built on top of
USB that allows GPIB-like communication with USB devices. From the user's point of view, the
USB device behaves just like a GPIB device. For example, VISA Write can be used to send the
*IDN? Query and use VISA Read to get the response. The USBTMC protocol supports service
request, triggers and other GPIB specific operations. USBTMC allows instrument manufacturers
to upgrade the physical layer from GPIB to USB while maintaining software compatibility with
existing software, such as instrument drivers and any application that uses VISA.
December 2019 44 LDC-3706 Series Laser Controller