Synching clock signals through SCXI-1302's and a network hub...

[Drexel-CECE]

Good afternoon all--

I am a electrical engineering major at Drexel University. We are working on a power systems project that utilizes a decent amount of equipment from National Instruments. Below is a description of one of our basic setups:

    - SCXI-1000 chasis

    - (2) SCXI-1327 Voltage Attenuators - One is used to pass our current and voltage measurements from the Hall Effect Devices (HED's) to the DAQ board in the CPU. The other is used as a pass through for our power supply circuit.

    - SCXI-1302 Terminal Block - Used to connect two signal conditioning units together (i.e. Master-Slave). This is where we synch the measurements. One signal conditioning unit is set as the master (or sending end), the other as the slave (or receiving end). Currently we can just have two units connected together, no more.

    - SCXI-1180 pass through - Sits under the 1302 terminal block

Here is my problem. What we would like to do is connect all of our signal conditioning units together. Like I said above, right now we can only connect 2, one as master, the other as slave. We want to be able to connect multiple signal conditioning units together so that one can be set as the master, and the rest as slaves.

Our first idea was to use a network hub. We would connect two conductors at one end of a cat5 cable directly to our 1302 terminal block (PIN 46 - PFI 7/AI SAMP CLK and PIN 24 - D GND), and then at the other end we would have a RJ-45 jack to plug into the hub. The idea would be that the hub would sync any signal connected to it. So if we wanted to connect Stations 3 and 5, all we would have to do is plug them into the hub. 

This isn't working as easily as planned. When we connect to the hub, we don't get anything positive. I'm not sure if it has to do with the RJ-45 jack, the hub, or the NI equipment, or even the DAQ software. 

Can someone please help me try and figure out this problem? Any help would be greatly appreciated. Thank you.

-Ryan V 

[DavidMN]

Hi Ryan,

I am not sure I fully understand the need for the network hub (eg why do all the signal conditioning units need to be connected).  Would you please elaborate?

[Drexel-CECE]

Dave --

Sorry for the confusion. I didn't want to give out too much information and confuse everyone from the start.

We are using these units for a power systems lab. Our power system consists of 3 buses connected in a ring-type fashion. This means that each bus is connected to the 2 other buses in the system. For our lab, we need to take measurements at the end of EACH LINE. Doing the math you'll find that we need a total of 6 measurement stations. That means 6 signal conditioning units containing the equipment listed in my first post.

Normally, we only have 2 buses, 1 line, which means only 2 signal conditioning units. In that case, we can just hardwire them together directly to synch the measurements. However, with our system, we would like all 6 points to be synched together to get the most accurate results. This leaves us with two choices: we either have to hardwire 6 signal conditioning units together, or find some way to "plug and play" each signal conditioning unit as needed. We don't want to hardwire because the lab we are working in performs various other experiments that doesn't require the sychronization we need. The network hub was just an idea we thought might work, but we were wrong. That is why I came here, to see if I could get any help from others who have done anything like this prior. Hope this clears things up.

Thanks.

-Ryan

Message Edited by Drexel-CECE on 04-09-2009 10:41 AM

[John_P1]

Hi Ryan,

Welcome to the NI Forums!  Have you considered daisy chaining the chassis together?  In this mode the channels of your chassis would be multiplexed into the same DAQ device (so would be slightly out of phase), but I would imagine this would yield better results than indepdendently running the multiple chassis from different DAQ devices.

If you are controlling the chassis from multiple DAQ devices, you have to share clock signals between them to achieve synchronization.  You mentioned you have done this using the 1180 to access the PFI lines of your DAQ devices.  I am not sure about the Hub, my guess is that a clean signal is not making it through for whatever reason--can you probe the line to verify whether or not you are receiving a signal?  If you could find a way to route the signals cleanly through a custom ethernet solution then this should work (the PFI lines require TTL signals).

Another option would be to use the RTSI bus on your DAQ devices, which would save you the hassle of physically connecting/disconnecting multiple SCXI chassis together.  You can route signals over the RTSI bus the same way that you are using the PFI lines. 

-John