SCSI Speed with Multidevice per chain

[xDeLiRiOuSx]

I have an Adaptec 2940UW with internal 68pin and internal 50pin. I am currently have a Plextor 12x10x32x Ultra Scsi CD-R drive on the scsi chain. My question is that if I added a Iomega SCSI-II zip drive on to the chain will it slow down my plextor CDR?

IF so, does anyone know of a good PCI 50pin scsi card that does is NOT bootable (I hate waiting for the scsi devices to show up on the bios - it wastes too much time).


Thanks
David

 

[Peter]

On SCSI, slowdown happens only if you add an UW-or-below device to an U2W-or-above channel, or when simultaneous use of devices exceeds the channel's bandwidth.

Ex. You have an UW channel there, using only Narrow transmission, that gives you 20 MB/s. The 32x Plextor consumes no more than 5 MB/s, leaving 3/4 of the SCSI bandwidth unused - so even if that ZIP drive uses the worst thinkable SCSI interface, there won't be any slowdown.

With Ultra-SCSI, mind the cable length, and don't use 25-pin external cables. These have too poor shielding.

Besides, you should be able to turn the boot support off in the Adaptec's configuration menu.

regards, Peter

 

[mastertech01]

Attaching a SCSI-1 device to a system with a SCSI-2 host adapter and several SCSI-2 devices already attached will not hurt over-all performance significantly unless it doesn't handle disconnect/reconnect well. This assumes that the host adapter keeps track of protocol options separately for each target device. Some people have the idea that attaching a SCSI-1 device to a SCSI-2 bus will cause the entire bus to run at SCSI-1 speeds. This is not true.

As for waiting for the detection to show in the boot process, that feature can be disabled in bios. There are several non bootable scsi controllers that will do the same because they have no boot bios, but most I have found are SCSI 1 controllers. Bear in mind though that at some point your SCSI buss MUST be scanned and drives detected or they would not show in your operating system, so there will always be some delay to boot up. I have 3 SCSI controllers so my wait can be substantial.

 

[xDeLiRiOuSx]

Thanks SCSI experts... I WAS under the impression that a attaching a SCSI-I device on a SCSI-II ( actually an Ultra SCSI -plextor) would make my chain run at SCSI-I speed. Thank you for correcting me. Now all I have to do is buy a longer SCSI cable so I could hook up my ZIP and Plextor on the same chain (need it to have like 5 devices. hehe).

David

 

[xDeLiRiOuSx]

ONE more question. I have an ISA card that connects to the ZIP right now. It works fine. Should I move the zip to the Adaptec 2940UW and put it on the same chain as the Plextor. Or leave it the way it is. I am just afraid that the ISA legancy card (ACA-1510) will actually slow down my system. Is that true with any ISA card?

Thanks
David

 

[Peter]

Oh yes, ditch that ISA card. Among Adaptec's ISA offerings, only the 154x series did DMA, everything below is CPU driven programmed I/O. Apart from being slow on the SCSI side, they eat a huge amount of total system bandwidth while accessed.

mastertech is correct about the disconnect/reselect bit, but since your Plextor drive doesn't support that either, nevermind.

regards, Peter

 

[xDeLiRiOuSx]

Going to ditch that ISA card. Thanks.. So everyone agress that I should put the IOMEGA ZIP drive and the Plextor 12x Burner on the same scsi chain right? And since I don't have discconect / reconnect on either drive, it should be perfectly fine right?

Thanks
David

 

[Peter]

It'd be fine either way. Drives supporting disconnect/reselect can yield the SCSI bus to others while processing a command. It's a bandwidth saving feature.

Apart from the electrical <=UW vs. >=U2W threshold, drives on a single chain may be any level of technology. I have an ancient UMAX scanner and SONY MO (both SCSI-1, asychronous), FastSCSI CDROM and tape drive, and an Ultra-SCSI CDRW drive on the same chain. No problem - other than that while the scanner or MO are running, they stall the others. No data lost though.

regards, Peter

 

[xDeLiRiOuSx]

Hi Peter, Do you know anything about ZIP drives? Because My zip drive's physical eject button never worked. However ON the ISA card, I went into windows and clicked on "eject" disk and it would do it. Now when it's on the 2940UW (works Great) it won't eject.. Even if i put it on the chain alone (no cd rom). The options are as: every thing is set as yes.. Wide Negotiation - YES, discconnect - YES, Include in BIOS scan - YES, oh yeah I think "send start signal to device" is NO.

Please help me out, I can't get the disk out now (except via emergency eject).

David

 

[xDeLiRiOuSx]

Welp, I even disable "boot from remable scsi devices"

Now the options on the ZIP are as follows:
Initiate SYN Negotiation - YES
Max Transfer Rate - 40.0mb/sec
Enable Disconnects - YES
Intiate Wide Negotiate - YES
Send Start Unit Command - NO
Inside in BIOS scan - YES

Thanks
David

 

[xDeLiRiOuSx]

help?

 

[mastertech01]

David that eject should work with any controller. It sounds like a problem with software or the drive itself. That drive is asynchronous so it wont do any good to set for synchrounous transfers. You also cannot run it or the CD drive for wide negotiation unless the Plextor is the wide version and then if thats the case the Plextor would go on the wide cable. Max transfer rate is less than 10 on the zip. Try downloading the Iomega software for the zip and see if that restores the eject function. There may even be a jumper on the drive that disables eject button. You may even want to go into controller bios and reset all defaults, then disable bios scan (for hard drives).

 

[Peter]

Sync and Wide transfers are negotiated, folks! This means you let the adapter start the negotiation with its own maximum settings, and each drive's answer comes back reduced to what the drive can do. It is explicitly NOT required to operate the SCSI host adapter's configuration menu for this. All the performance enhancing features in SCSI devices are auto detected.

The limitation items in the SCSI configuration menu are for further limiting setups that don't work in "Auto" mode - mainly this is because of (fairly rare) drive firmware errors that screw the negotiations up, or too long cables where you want to force the entire chain back from Ultra to Fast.

regards, Peter

 

[xDeLiRiOuSx]

Okay, Thanks guys.. So That bites.. My zip drive still ejects via the ISA card, but doesn't do it with the Adaptec card. Kinda pissing me off. But looks like I have a faulty zip drive.

David

 

[Peter]

Not really ... if the Adaptec SCSI BIOS adopts the Zip as a BIOS supported boot device, then it won't eject unless the operating system unlocks it. This is a normal, and recommended, behavior for removables.

The Adaptec SCSI configuration menu certainly has a means of disabling use of removables as boot devices. That'll cure it.
regards, Peter