In my original posting I wrote:
> Can you (officially) mix 68 pin SCSIs diff and non-diff devices
> on the same bus?
>
> Will performance suffer if you mix them?
>
> Will performance suffer for diff devices if one terminated end of the
> ribbon cable is missing l8 connections?
>
> Does anyone know if the RZ28M and RRD45 are differential devices? They
> have 68 pin connectors but apparently that doesn't necessarily
> mean they are differential.
Thanks to three respondents who straightened me out on this. I mistakenly
assumed that having 68 pins qualified a device as differential, it in fact
only means that it is wide. I thought extra pins were needed for returns
on a differential bus, but it turns out existing ground pins are reassigned
as signal returns, requiring no extra connections. So in summary (ignoring
SCSI-3) there are 4 cabling/connection protocols of SCSI devices out there:
Narrow/Single (8bit/ground signal return) 50 pin
Narrow/Differential (8bit/individual signal return) 50 pin
Wide/Single (16bit/ground signal return) 68 pin
Wide/Differential (16bit/individual signal return) 68 pin
When you understand this you can see that one *cannot* mix single ended and
differential devices, even though the connectors are the same, unless you
have a converter (see Alan Rollow's and Peter Flack's responses). You *can*
combine narrow and wide devices on the same bus (but beware of 16 bit vs 8 bit
addressing conflicts as described below by Eugene Chu!).
Thanks for the quick (and definitive) responses, which follow.
-------------------------------
| From: chu_at_musp0.Jpl.Nasa.Gov (Eugene Chu)
|
| First of all, the 68 pin SCSI cable indicates it's a WIDE SCSI, not
| necessarily differential. A differential narrow SCSI bus can use the
| same 50 pin cable as a single ended narrow SCSI bus, just as a
| differential wide SCSI bus can use the same 68 pin cable as a single
| ended wide SCSI bus.
|
| You can mix wide and narrow devices on the same bus, but you can not mix
| differential vs single ended devices on the same bus. Just remember
| that the wide bus uses all 16 data lines to address bus targets, while
| the narrow bus uses only 8. So, if you have a wide device whose lower 3
| address bits match those of a narrow device, you will have a bus
| conflict. Unfortunately, none of the current DEC UNIX versions will
| address beyond device 7 anyway. The benefits you get with the wide SCSI
| is higher peak throughput on the bus, and the possibility of setting the
| controller to a high address (15) and putting up to 8 devices onto the
| bus in the lower addresses instead of 7.
|
| As far as performance on the bus, it depends entirely on the device as
| to whether it will slow down other devices.
| ---------------------------------
| From: alan_at_nabeth.cxo.dec.com (Alan Rollow - Dr. File System's Home for Wayward I
| nodes.)
|
| The best way to mix differential devices and non-differential devices
| is to put the non-differential devices on the other side of a
| converter; a DWZZA for narrow devices and DWZZB for wide devices.
| I don't think differential is a performance characteristic, so
| there shouldn't be any performance problems.
| ---------------------------------
| From: flack_at_rtp4me.ENET.dec.com (Peter Flack)
|
| The Rz28M (and I think the RRD45 as well) is a single-ended SCSI device...
| You cannot directly mix single-ended and differential - they use completely
| different electrical signalling mechanisms. Digital offers two devices
| that convert single-ended to differential signals - the DWZZA (for narrow
| single-ended) and DWZZB (for wide single-ended)
|
---------------------------------------------------------------------------
Ted Asocks tra_at_ucolick.org
Systems Administrator VOICE: (408)459-4020
UCO/Lick Observatory FAX: (408)426-3115
Received on Fri Apr 19 1996 - 19:21:32 NZST