SCSI is a very large standard, covering several different standards, and hasn't been helped by the older terminology that was used such as "fast-20, fast-40, ultra" and so on.
Its that comprehensive that it is difficult to give a precise, easy-to-understand explanation. And you need to have an appreciation, to avoid buying useless equipment, or equipment that is under/over specified for the PPC.
Ok, the SCSI bus comes in two flavours, narrow (8bit, uses a 50pin connector) and wide (16bit, uses a 68pin connector).
Bus speed (ignoring pre-SCSI-2) was 10Mhz (Fast-SCSI2) or 20MHz (Ultra-SCSI3, aka ultra-SCSI), giving max bandwidth of 20MB (ultra-narrow, 8bit 20MHz) or 40MB (ultra-Wide, aka UW, 16bit @ 20MHz).
Narrow SCSI supports a max of 7 devices per adaptor/chain, wide supports a mx of 15 devices.
Narrow SCSI is "obsolete", it has a max bus bandwidth of 20mb/sec (8bit at 20MHz). The only hard disks you will find are old, pre-2000. However, the vast majority of other SCSI devices - CD/DVD, scanners, Jazz, etc - use this standard, SCSI-2.
Now, it gets a little confusing. In the late 90's Ultra-2 SCSI came about. Dont confuse Ultra-2 with ultra-SCSI2.
Ultra2 (U2)raised the bandwidth to 80Mb, still using a 16bit 68pin bus. The major difference was the signalling method. All previous versions of SCSI are termed SE, or 'single ended', U2 uses LVD - Low Voltage Differential.
U2 was superseeded by U160 (160MB bandwidth) and in turn by u320, 320MB bandwidth.
There is a further SCSI signalling standard, call DiffSCSI. This is a signalling variatoin on SCSI-2, essentially it is the same as ultra-wide SCSI but uses much higher voltages on the bus to allow for longer cabling runs. Do NOT ever Ever EVER mix Diff SCSI and LVD/SE devices; a diffSCSI card will BLOW every SE/LVD device on the chain.
The good side is, all SCSI is compatible, certainly downwards and according to spec, upwards (bar the exception above). This means that a modern, U320 drive will work on a ultra-wide SCSI2 adaptor, and an old 8bit hard disk will work on a modern U320 controller. Usual caveats apply, though!
It is also possible to mix wide and narrow devices on th chain, ie an 8bit optical drive on an 16bit chain. But unless one does this very carefully, bandwidth may drop; SCSI adators test the chain on startup and negotiate the highest common bandwidth - ie they set it to the slowest device on the bus. This directly affects you.
Ok, the PPC uses an ultra-wide SCSI interface, based on the Symbios 53C770 chipset, max 40MB/sec bandwidth with 16bit transfers.
So, the type of drives you are looking for are ultra-wide SCSI, (aka UWSCSI disks). These used to plentiful on ebay, around a few quid for a 9gb drive. They are old, "slow" (7200rpm), and max transfer rate is limited to 40MB. But ideal for the PPC. U2 or U160 drives will work, without problems, but again be limited to the 40MB bandwidth of the bus. However, you might find them 'faster' as more modern drives tend to have larger cache and lower latency (which is the real key to speed in any hard disk). But the difference will not be blinding in use, so when considering buying drives bear that in mind if any additional cost is worth it.
Adding a narrow device to the bus can drag the whole bus down to 8bit operation, and adding a non-ultra (ie 10MHz) device dragged the bus down to 10Mhz operation. Depending on the SCSI BIOS, this can be avoided; some cards (and I *dont* know if the PPC/Symbios 770 chipset can do this) had the abillity to set this on a per-device basis; ie when talking to a UW device it used 20Mhz, when talking to an non-ultra device it used 10Mhz).
Termination.
This probably the most common cause of problems, even with experienced SCSI users. The bus needs to be terminated; if you imagine the electical pulses going down the wire, what happens when the reach the very end? A terminator, in very simple terms, soaks them up and prevents them reflecting back down the bus. Thus a SCSI chain needs a terminator at both ends. Most cards have an inbuilt terminator on the card.
A diagram would look like this, with a card, 3 devices, and a 5 header cable:
|----|----|-----|-----|
CARD dev1 dev2 dev3 <-dev3 needs terminator set
using resistors or jumper
|--- is the cable, with the | being a connector
(I'm ignoring cards with internal and external terminators, as its not relevant to the PPC)
The old 10Mhz SCSI used passive termination, but Ultra2 and higher needs an active terminator. Ultra-wide is in the middle. Personally, I used active terminators full stop, as even some old narrow 10Mhz devices really wanted active termination. Pratically every pre-U2 device had the option to be terminated; old SCSI boards and HD's used resistor packs, 'newer' older drives and optical drives used a jumper. I used to terminate the last device on the chain, and fit an active terminator. So my chain would look like this
|----|----|-----|-----|
CARD dev1 dev2 dev3 AT <- active terminator
(R/J) <- term set on device.
Mixing devices.
Don't. Really, if you can avoid it, don't do it. Only put UW devices on the UW chain, and obtain a second, narrow (eg any Amiga SCSI card) for optical drives, etc.
It can be done, one can mix ultra-SCSI and non-ultra-SCSI devices (everything will work at non-ultra speeds) - this is the most common occurence when one adds an optical drive.
One can add a narrow device to a wide chain. Now, this *shouldnt* result in everything working at 8bit. However, it is important where one places the narrow device - the first device after the card, or the last? This is wholly dependant on the card, although in general it is the last device. However, bear in mind that narrow devices *tend* to be non-ultra.
Devices and bus speed question.
As the bus is limited to UW SCSI (40MB), then adding a 50pin device will only affect if
1. It is non-ultra - whole bus will slow to 10Mhz. HD speeds greatly impacted
and/or
2. There is issues with placement; if wrong position, the bus will have problems communicating with the wide devices. Impact from severe (speed decreases) to show stoppers - the W drives cannot be seen by the card, or the whole chain locks.
Ok, the majority of SCSI optical drives were non-ultra. Simple because the write speed ofthe drive did not require >10mb bus bandwidth. In 2002 Plextor did bring out an ultra-SCSI CD burner, I had one, but cant remember which model. I also beleive that they bought out a Wide version, presumably ultra-wide, but although I heard about it I nevr sawone for sale in the UK. Else I would have had one!
Frankly, there is no call for high-speed SCSI CD burners. Back in the mid-late 90's it was the "only true way" to "reliably" burn CDs. But SCSI commands a premium, and with PATA technology improvements such as MWDMA, and then UDMA bandwidth increases, plus more PATA connectors on boards, ATAPI burners took the lead. So SCSI burners fizzled out. The Amiga really benefits from them, but as an old niche machine there is no compelling market for them. Ebay etc is your friend. I had a stock of SCSI optical drives, from 4-16x, simply for server use (an 8x SCSI CD installs Netware much faster than a 32x IDE CD, for example). I might still have a SCSI CD burner somewhere, but I found them superflous. All my PCs use U160/U320 systems, with maybe just one having an Adaptec 2940 for some legacy SCSI stuff (CD jukebox, DLT drives). I'm not going to handbrake the bus with an 8bit burner, when I have the PATA ports available and brand new DVD burners available for under £20 now (and only circa £40 last time I built a mean machine).
Anyway, this is long, given my wrting you are probably more confused, so I'll quit.
In short, ask here or over at EAB before buying any drives etc, and we'll see what we can advise. If you get stuck, shout. And if you get really desperate, I have a few suitable spare drives and even Amiga SCSI cards. Heck, I got loads. But I'm not on a sales pitch!
G'luck. If you get really confused, and live in UK, drop me a PM with a phone number.
