mvme5500: data transfer rate to and from vme space

Till Straumann strauman at
Tue Oct 23 20:16:25 UTC 2012

Are you really using VME BLTs (or only repeated single-address cycles,
e.g., the CPU doing a loop - this can be really slow, especially if the 
is configured so that the bus has to be re-arbitrated for for each cycle)?
How do you do those block transfers - with the DMA engine, I suppose?

If it is too slow then you might want to use a bus analyzer to find out why.
In some cases the VME card you are transferring data to or from
(i.e., not the mvme5500) may be slow. Since VME is an asynchronous bus
speed is limited by the slowest device participating in the transfer.


On 10/23/2012 09:37 PM, rwas wrote:
> Hello,
> I've done some timing measurements on block transfers of data from the
> vme space to
> local memory space. My effective transfer rate is about 2mbytes/sec. In
> theory, the vme
> backplane can handle about 20mbytes/sec. I've checked this for various
> block sizes and
> the transfer rate scales roughly proportionally with size.
> word size: 32bit (transfers are 32bits at a time)
> block size: 256bytes: 150uS
> block size: 1040bytes: 530uS
> block size: 3032byes: ~1.2mS
> addr space: 32bit
> rtems: 4.9.3
> Help appreciated.
> Robert W.
> _______________________________________________
> rtems-users mailing list
> rtems-users at

More information about the users mailing list