GPIB-USB-HS slow to respond

[Steve289]

I am new to GPIB controlled devices and a problem that I have encountered is that there is a lag between the time I write a command and when the device actually responds.  I am using a GPIB-USB-HS controller to control a Xantrex XPD-30 power supply.  I need to do a voltage sweep from 8 to 14 volts, 10mV increments in a time period of 6 seconds.  This works out to 10mS between increments.  I was unable to use the GPIB controller to do this because the amount of time that elapsed between the command being written and the power-supply responding was to long, forcing me to use the analog outputs on my DAQ card and the Analog inputs that the power supply also has.  Is this a normal characteristic of GPIB devices.  I am aware that there is some time spent on receiving the command and the GPIB responding back saying that it was received properly.  But I thought that one of the benefits of GPIB was the speed at which commands can be executed (7.8MB/s).  Any input on this problem would be greatly appreciated.

Thank You

Steven.

[LuI]

Steven,

there seems to be some mismatch in your process understanding. Let me explain some technical background.
1. You write some commands to the Xantrex. This means finally sending a data packet via USB to the GPIB controller. This data packet contains the instruction to send the command to the Xantrex.  As USB is a packet based protocol, there is often a delay between receiving some data to be sent and the actual sending of the packet. USB data packet have a somehow fixed size. 'Incomplete' (e.g. not filled) packets are usually delayed to save bandwith by accumulating complete packets. There is, however, a timeout, when even those packets are sent, but this timeout is an effective delay. We have seen delays of up to nearly a second.
2. After all, the GPIB part of transmission is quite quick and may reach the throughput you mentioned.
3. When the Xantrex has received the command, ist must execute some code and send a respond back. This takes some time, which depends on the firmware and probably the command you requested to be executed. Even a simple voltage increase may take some 10 to 100 ms or more until the response is send.
4. The response goes first through GPIB transmission - see 2.
5.The GPIB controller translates the response into a USB data pvcket - see 1.

You may gain some speed when using a different GPIB controller - especially one that directly sits in the controlling PC (usually a PCI- or PXI-based one). But I am not sure if this will solve your problem.

A much better way is to check if your Xantrex device supports such sweepings by itself. If so, you instruct it to perform the required action and trigger it to start. After that, there is no more communication needed in order to fullfill the sweeping task. And such those delays are no longer affecting the process.

Greetings from Germany!<br>-- <br>Uwe

[Ibrahim330]

Hi Steven

Please can u help me a bit, I am trying to have a voltage sweep (from -10V to +10v)with a Keithley 6487 to get the VI Characteristic of an LED. I am new with LabView, I have tried to use one example to get the VI but i can increase the voltage manually from the front Panel Diagram. I tried also the IVsinglescan_transfercurve.vi but it is not working, please I need ur help

Regards,

Ibrahim

Germany.

[Dennis_Knutson]

You asked a similar question here. You should have responded there (or better yet, create a new post). Your question here has nothing to do with the original question and just confuses things. As I said in your first post, you need to use a loop which is a basic function in LabVIEW. There are links to LabVIEW tutorials at the LabVIEW Learning Center and numerous shipping examples that can help you get started. If you continue to struggle, post a new question to the forum with additional details and include the code you've already written.