Connecting USB to D-SUB 15 pin port of PCI 1426

[sandeepchinni]

Hi,

I have asked this question as a sub question in one other thread which I started, but it seems complicated enough that I need to start a new thread for it.

I need to connect an external device (GPS) which has a USB port to the D-SUB 15 pin port of the PCI 1426 frame grabber. I know that the USB port has a power and ground pins and two data pins. I also know that USB sends out data in differential voltage. But I am not sure whether the frame grabber can detect differential voltage.

My requirement is to send triggering pulses to the GPS device and read the responses given by the GPS device. I am thinking of using the TTL I/O 1-4 pins of the 1426 for this.

Can any one tell me how this interfacing can be done?

Thanks & Regards,

Sandeep

[Dennis_Knutson]

Frankly, I don't see where you have any chance at all of getting this to work. How are you going to generate all of the complex messaging protocol of a USB connection? Do you have any information from the vendor about the USB port and the protocol used? Why don't you connect the device to the USB port of the pc?

[sandeepchinni]

Thanks Dennis.

Apparently I have under-estimated the complexity of the problem!! This is the first time I am trying to interface the D-SUB 15 pin port.

The device gives out information in TSIP (protocol) format. The reason I am not preferring to connect the device to the USB port of the system is the speed/latency constraint. I need very high speed and low latency and I thought that using the D-SUB port would provide this.

a) If I do away with the part of sending the signals to the GPS and only read the messages from the GPS, would the interfacing be possible?

b) One other idea that I have thought about is the use of a USB to Serial port converter. If I connect the USB port of the device to a USB to Serial port converter cable and connect the data pins of the serial output to the TTL pins of the D-SUB. Would this be possible? (In this option also I am alright with the idea of doing away with sending triggering signals to the GPS which can be operated in auto output mode.)

Please let me know your thoughts.

Regards,

Sandeep

[Dennis_Knutson]

There are at least a couple of problems that I see. First, the instrument does not have a serial port. It has a USB connection and the only way it would appear as a RS-232 port is if the driver (inf file) you install on a pc makes it emulate a RS-232 port. Without a driver installed and the instrument connected to a pc, I don't know what the USB port on the instrument would do.

Second, even if the instrument had an actual RS-232 connection, you would have to emulate a RS-232 connection with the digital I/O lines. Correct me if I'm wrong but isn't the digital I/O software timed? I don't see how you could possibly generate the precise timing required for RS-232 at even the lowest baud rates.

The software timed digital I/O would also make it very difficult to capture the raw signals being put out by the instrument (if it would even generate the signals without a connection to a proper USB port). With software timed digital I/O, you would probably have at best, an acquisition rate of 1kHz. The instrument almost certainly streams data at a much higher rate.