SCSI and termination.

[markyb86]

1) Can someone explain to me Termination on SCSI and why it is necessary?

2) Does an iomega ZIP drive need to be terminated to work properly with a Mac Plus or Macintosh SE?

Thanks in advance. I have only used internal SCSI devices in the past, However, I keep hearing about termination/terminators, and my google-fu hasn't helped me (or was a little too upper class tech for me... sadly I'm a microsoft certified professional and A+ certified, but I don't recall much about SCSI from A+ and microsoft doesnt help much with apple :beige: )

 

[CC_333]

Hi,

I can't really answer 1) very well, but for 2), yes, you do need to terminate for use on a Plus (but not an SE/30 or Classic II, for some odd reason). This is especially important since the Plus has a kind of weird SCSI implementation (because it doesn't fully conform to the SCSI standard, which was finalized after the Plus first shipped) and apparently isn't very tolerate of improper wiring/termination of peripherals.

If you're mindful of some of it's oddities, however, it should work just fine.

I hope this helps!

c

 

[markyb86]

Thanks for the reply! Would this work then?

http://www.ebay.com/itm/IBM-SCSI-Terminator-25-Pin-72X6365-17G2645-/280918820475?pt=US_Drive_Cables_dapters&hash=item416811067b

and am I correct guessing I need one for the left port (so a female plug?)

(I would know the answer to this had the cable came with it lol.. the guy is sending it tomorrow though.)

 

[CC_333]

Hi,

It looks as though you have a ZIP plus there. Mine is SCSI only, and it has a terminator on/off switch.

I don't see one there. Perhaps it's automatic?

You might want to read up on the ZIP plus drive and see if it has termination, and if so, how it's activated/deactivated.

c

 

[markyb86]

OK found it on the iomega site. AFAIK, if hooked to an SCSI port, it must be the only device on the chain. So I'm guessing its equipped with whatever it needs....

Now I am just confused on why there are two ports. It seems that Parallel and SCSI share the same one? There is really a lack of info on the site. I found way more info on how to get it working in linux than info about the device itself..

Oh well. If my guess is correct then I should be good to go.

Thanks!

 

[phreakout]

markyb86,

The reason why there are terminators on SCSI is due to the fact that there is such a strong signal sent down the wire. When it reaches the other end, if nothing is there, it will bounce the signal back to the source (it echoes). It's almost like how echoes reflect off of the sides of mountains. Without a terminator, you will experience a variety of symptoms from "ghosting" (the icon for that SCSI drive appearing and disappearing repeatedly), "mirroring" (the icon shows up twice, one below the other) or no device at all. The terminator acts like a shock absorber for the entire chain.

One thing to remember about SCSI is that you can "daisy-chain" devices together:

Mac's SCSI port<===>SCSI device<===>SCSI device<===>SCSI device<===>etc.

This is why there are at least 2 SCSI ports on a SCSI device. SCSI, offers up to a handful of devices you can link together. But also in order for a successful set up, you must have assigned one SCSI ID number per device. Here's how it breaks down:

SCSI ID #0: Reserved for the Mac's first internal hard drive

SCSI ID #1:

SCSI ID #2:

SCSI ID #3: Generally reserved for internal and external CD drives

SCSI ID #4:

SCSI ID #5:

SCSI ID #6:

SCSI ID #7: Reserved internally for the Mac (not allowed to use)

Regarding SCSI, according to tech writers David Pogue and Joseph Schorr, there are "The 3 Rules of SCSI":

1.) Termination:

Typically, the Mac's internal hard drive will have its terminator enabled by default. Externally, regardless of what the SCSI device is, you must have a terminator installed on the last device on the chain. Now, some external SCSI devices may already have termination installed and (depending on the manufacturer) there may be a switch you can adjust to turn the terminator on or off. Also, some SCSI devices may have "active termination" built-in, where a circuit will turn on or off termination automatically as needed.

2.) SCSI addressing or ID number:

Each SCSI device must be assigned their own number and you can't share that ID number with another SCSI device. The physical order of devices can be independent of their ID numbers and the last device on the chain doesn't have to be the highest or lowest number assigned. However, Apple recommends you give higher numbers to the devices you use most frequently and lower numbers to devices used less frequently.

3.) Cabling:

Keep the cables as short as possible. You're allowed up to 18 feet (6 metres) total, but you must subtract from that figure the amount of cable inside an external device. Finally, the quality of the cables make a huge difference in the successful maintaining of SCSI. So if you can, buy cables from a name brand you can trust.

73s de Phreakout. :rambo:

 

[markyb86]

Wow, thank you for that information!

I am way, way too used to simple things like master/slave IDE... hehe.

 

[phreakout]

Your welcome. I guess that's why IDE replaced SCSI as a standard for hard drives. Technically, they both are so alike. But who wants to monkey around with confusing DIP switches and settings for these kind of drives?

73s de Phreakout. :rambo: