PCMCIA-485 card doesn't run at 921kbps

[jmalos]

We have a PCMCIA-485 card sending serial text files to our Comtech CDM-600L satellite modem. The modem accepts RS422 and EIA530. We have set the laptop's port control in Windows XP to allow for 921kbps and we see this setting change in the NI Measurements & Analysis window. We are running the latest version of the NI drivers (3.5).

We have written a custom program to open up the port and ship the text file out at the 921kbps. The modem is set to run at its top speed of 5Mbps. When we sent a 5Mb file, it took 80 secs which equates to approximately 600kbps. However, we see this same speed when using the NI VISA test panel as well.

Any idea what is holding the speed down?

[Mr.F]

Hi,

Just to be sure I believe you meant to write 8 seconds, right? If that is the case then your measured speed is to be expected. At “full” 921kbps the 5 MB file would take an ideal 5.43 seconds. The key point is that VISA transmits information in frames, with start bits, stop bits, parity bit, etc. The actually data that you are transferring may only be 5 out of the 8 bits in the frame. This difference would account for the increased time that you are seeing. I hope this information helps.

[JasonSmith]

I am assuming that you meant you transferred a 5 MByte file, and not a 5Mbit file? If this is the case, then the theoretical minimum time would be about 57 seconds. The reason you are not able to achieve this transfer rate, is that the 921600 baud rate is indeed the rate the data is being clocked out at, but there are gaps in the data where the FIFOs on the hardware are being reloaded.

Unfortunately this is an older device which doesn't have the larger buffers of many of our newer products. The smaller buffer combined with the legacy ISA-like interface utilized over PCMCIA makes it difficult to keep the bus saturated.

One thing that you can check is to ensure that the FIFO fill level (available on the advanced tab in MAX) is set to the highest level possible, which I believe is 16 for this device. If this is already set to 16, then you will probably not be able to achieve more than the 600 kbps you are seeing now.

-Jason S.