At 14:36 -0600 12/07/2002, Andy Ball wrote:
>Hello,
>
>   BS> The on board SCSI is narrow (10MB/s, 5MB/s external).
>
>   DG> It's also probably worth noting that the MESH
>     > controller was configured to run at slow external bus
>     > speeds, but that its clock speed can be changed in
>     > software to improve performance if the bus is clean
>     > enough to handle it.
>
>Is that how it does 10Mb/S internal and 5Mb/S external (if
>that's right)?  Or is it simply clocked at 5MHz and the
>internal segment is 'slow, wide' (which seems unlikely)?

The SCSI on the Beige is 5 MB/s, period.   The internal SCSI 
connector and the external SCSI connector are both on the same bus. 
That bus is unenhanced SCSI-II (5 MB/s).   There is no SCSI faster 
than 5 MB/s on the Beige G3 without adding a PCI card (except for 
possible hacks such as David mentioned).   The Beige G3 SCSI is 
controlled by the Heathrow chip, which apparently contains a block 
based on the MESH chip.

The PowerSurge machines (7500, 7600, 7300, 8500, 8600, 8600 Enhanced, 
9500, 9600, 9600 Enhanced, PowerTower Pro, PowerWave, Daystar 
Genesis, Umax S900/J700) have two separate SCSI busses.

The one which appears as SCSI Bus 0 is controlled by the Apple MESH 
chip (which may be based on the NCR/SYM/LSI 53CF96) and has only an 
internal 50 pin IDC (ribbon cable connectors are called IDC) 
connector.  It is a Fast SCSI-II bus and supports 10 MB/s theoretical 
transfer speeds.

The SCSI bus in PowerSurge which appears as SCSI Bus 1 is controlled 
by the AMD 79C950 chip (CURIO Chip).   This bus has both an internal 
50 pin IDC connector and an external DB25 connector.  Both these 
connectors are on the same bus.   There is circuitry to autodetect 
whether termination is needed at the motherboard (between the two 
connectors, e.g. if there are only internal devices or only external 
devices on the bus).   This bus is Unenhanced SCSI-II and supports 
only 5 MB/s theoretical transfer speeds.

So, on the older PowerSurge machines there is a MESH SCSI bus which 
is internal only and supports 10 MB/s.  On the Beige G3 the only SCSI 
is MESH based, but it only supports 5 MB/s and has both internal and 
external connectors.    I think this change in what MESH does, and 
step backwards in performance also led to a lot of confusion about 
the Beige G3's capabilities when it was released.

Jeff Walther