New SCSI Host Adaptor

discuss both original and modern hardware for the bbc micro/electron
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KenLowe
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Re: New SCSI Host Adaptor

Post by KenLowe »

Ok, so I think this is looking much better now.

[1] Yellow: D0 between buffer and drive
[2] Cyan: D0 between CPLD and buffer
[3] Magenta: RST between CPLD and buffer
[4] Blue: I/O between CPLD and buffer

Triggering on rising edge of RST.

On the CPLD side, I can see that D0 drops to either 0.4v or 0.2v (depending on status of I/O), which is a good bit lower than the 0.6v / 0.7v I measured previously.
On the drive side, I can see that D0 drops to about 0.2v. Again this changes a little bit depending on which device is writing to the data bus.
Monitoring CPLD D0, Drive D0 and CPLD I/O on RST
Monitoring CPLD D0, Drive D0 and CPLD I/O on RST
Zoomed in to see how D0 responds when I/O changes
Zoomed in to see how D0 responds when I/O changes
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KenLowe
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Re: New SCSI Host Adaptor

Post by KenLowe »

That's me now received some final SMD based buffer & termination boards from JLCPCB for use with 80 pin Seagate SCSI drives. This final version of the board is the correct size and allows the SCSI host adaptor to sit nicely on top of the drive. I've soldered one up, and given it a quick test. So far, so good 8):
Comparison of prototype board and final board
Comparison of prototype board and final board
Final board connected to drive and SCSI host adaptor
Final board connected to drive and SCSI host adaptor
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KenLowe
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Re: New SCSI Host Adaptor

Post by KenLowe »

So, after much stalling, everything should now be available on GitHub:

https://github.com/kgl2001/BeebSCSIHostAdaptor

I just uploaded everything via the web interface, so I'm not entirely sure I've done it right. Let me know if anything doesn't look right, and I'll try and fix it. I'll also provide a bit more information in the ReadMe file about the various folders. I also intend to keep a list of verified compatible SCSI drives. So far, the following 3.5" (1" high) drives are confirmed working. If others are confirmed working, please let me know and I'll update this list:

50 Pin Drives - Requires 50 Pin Connector Board to connect drive to host adaptor
  • Fujitsu M2681SAM/U
  • Fujitsu M2682SAM/U
  • Fujitsu M2684SAM/U
  • Seagate ST31200N - *BYE command doesn't work for me
  • Seagate ST34572N
80 Pin Drives - Requires 80 Pin Buffered Connector Board to connect drive to host adaptor
  • Seagate ST34371WC
  • Seagate ST34571WC
  • Seagate ST34573LC
  • Seagate ST39173LC
When selecting a drive, there are a few important factors that you need to consider:
  • The drive must have the option for Parity Checking to be disabled. If this feature is available, then it is usually done via a jumper on the drive PCB. Check the drive documentation.
  • The drive will normally come formatted to 512 byte sectors, but the Beeb expects drives to be formatted to 256 byte sectors. The drive therefore needs to be reconfigured for 256 byte sectors. This can be done via software commands from the Beeb, but be aware that not all drives support this feature. Again, check the drive documentation.
  • The maximum space on the drive that the Beeb can access is limited 512MB. If the drive is bigger than 512MB, then any additional space will be unused, and will be inaccessible.
  • Although ADFS can support upto 4 drives (LUNs), it is usually only possible to connect a single drive to the host adaptor at any one time. This is because the more modern SCSI drives only support a single LUN.
The Fujitsu drives were my original drive of choice. However, I am now struggling to see past the Seagate drives (even though most of them are 80 pin and need the buffered connector board). They're easier to come by, and their Product Guides are very good. They provide all the information you need about parity and byte sectors. Please check the documentation before purchasing a drive! Not all (Seagate) drives are equal!
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