Bitrate question - IBM 3740 Format

[cube1us]

I am now working with the HxC SDCard Emulator on an Altos 8000-2. It uses a FD1791 chip, with a separate external data separator. I developed my own cable, which I have done before (e.g., to hook up a Catweasel board to 8" drives to image disks).

Restore, seek, etc. work fine, but when I go to read, I get a record not found error from the controller - it cannot find the sector.

I am using the same settings I used to get 8" emulation on the 5" connector of a Cromemco machine using a Cromemco 4FDC which uses an FD1771 chip, using its internal data separator.

In comparing what I see from the HxC unit to the specs for real 8" drives, I noticed something. All the real drives produce negative going pulses for each flux transition of 200-250ns. However, the HxC does not do that - it just generates transitions. I *think* that will work OK, but am not sure.

But as I looked at the raw data coming from the HxC, another question occurred to me: that of bitrate. The 8" drive manuals quote a bit rate of 250Kbits/s - but this is the *data* rate. If one sent all one bits, this would essentially double to 500Kbits/s. Which do I specify for the HxC? The bit *cell* rate (250 Kbits/s - 4us for each cell), or the bit *transition* rate (which would be double the cell rate for 1 bits)? This is the main question for this post.

I hunted around the forums and got somewhat contradictory results, hence the question.

The poster at

http://torlus.com/floppy/forum/viewtopi ... rate#p7361

seemed to have success with 500 Kbits/s, but I had previous success at 250 Kbits/s - but that was using a 5" connector, using the FD 1771 internal data separator, so I may have in effect been emulating a 125Kbit drive, I suppose, albeit with 77 track, etc.

Thanks in advance.

[cube1us]

Never mind. I found that reloading the image with a bitrate of 500,000 resulted in success.

[Jeff]

Never mind. I found that reloading the image with a bitrate of 500,000 resulted in success.

Ok good !

The bitrate in the HxC is more pulse rate. So for a data rate of 250kbits/s in FM, you need to double it to add the clocks pulses/bits.