SCSI termination and internal/external drives

[PacificState]

If I have an SE/30 with an internal SCSI drive, then the drive should have termination enabled. However, if I then add an external drive - do I have to disable the internal drive's termination and add it externally (replacing it with a terminator when the external drive is not connected), or leave termination off the external and rely on the internal? Thanks!

 

[cheesestraws]

You need to have termination at both ends of the bus, because those are the points where signal reflection happens. So both points where the bus stops as opposed to going through to another device. If you only have an internal, I believe that's handled by the LB, but if you have stuff going 'both ways' (thinking of the Mac in the middle) you need explicit termination on both ends.

 

[mdeverhart]

SCSI buses need to be terminated at both ends. Since the SE/30 only has one bus that’s shared between the internal connector and the external connector, you should always leave the internal drive terminated. When you connect an external drive, that drive should be terminated as well. If you have multiple external drives, only the last one is terminated. (Similarly, for Macs that have multiple internal drive bays, say a HDD and a CD-ROM, only the last device on the internal cable should be terminated, but the same rules apply for the external port - terminate the last device in the chain).

 

[tomcrete]

I'll see if I can locate my source but I recall recently doing some research on the Mac Plus and external drive I dug out of my closet. I read that the Plus _requires_ a terminator on the SCSI connected eternal drive but later Macs didn't actually require it. I'm still trying to locate the one or two terminators I know I have somewhere but the external drive does spin up but doesn't seem to be recognized by the Plus. I've just dug a Mac LC out of the closet and will try that external drive without a terminator on the LC.

 

[cheesestraws]

later Macs didn't actually require it

all SCSI buses need terminating at both ends. Sometimes you might get lucky with certain combinations of stuff and it might work when the spec says it shouldn't. But all SCSI buses require termination.

 

[PacificState]

Thanks. I'm still kind of confused, to be honest, after looking at the SE/30 schematic. I'd assume that the Mac itself would represent one side and be terminated, but the 50 and 25 pin share the same signals. This makes it conceptually easier (say) if you have a Mac with no internal drive and a (literal) chain of external devices (the last one would be terminated), but harder for me to understand if there are an internal and external - it seems to me that you'd need internal termination off and the external device terminated, except that would be a pain if you only plugged in the latter occasionally.

Surely I'm missing something, though?

 

[cheesestraws]

I'd assume that the Mac itself would represent one side and be terminated, but the 50 and 25 pin share the same signals

I think you're confusing the logical structure of the bus (where the Mac is at the top and everything else is underneath it) with the electrical structure of the bus (where the Mac is just another device with two scsi cables connected to it). Termination is not about SCSI as a data protocol, it's about how the electrical signals themselves travel down the wires, so it's where the wires go that matters, not what devices are talking to what. You terminate the points where the bus cables stop.

the 50 and 25 pin share the same signals

Think of the chain like this:

[internal drives] <-> [mac logic board] <-> [external drives]

The Mac is in the middle of the chain, not at one of the ends. The bus doesn't end there, it passes through. So the Mac is just a SCSI device with two ports like any other. The "ends" of the bus are the devices that only have one cable plugged in; the middles are devices with two cables plugged in.

So, essentially:

• If you only have an internal drive, the internal drive needs a terminator; the Mac can deal with the termination of the other end of the bus because it's at the end. ([internal drive] <-> [mac logic board]).
  
  
• If you only have an external drive, the external drive needs a terminator, the Mac can deal with the other end ([mac logic board] <-> [external drive])
  
  
• If you have both, you need to terminate both ends; the Mac is no longer at one of the ends of the bus, so it can't terminate anything ([internal drive] <-> [mac logic board] <-> [external drive])
  
  
• If you have more than one on the internal or the external bus, you terminate the last one on each end.

This is literally about where the cables stop. Nothing more, nothing less. If the cable doesn't stop, adding termination there won't help and may cause harm.

And obviously this doesn't apply in quite the same way for Macs that have two distinct SCSI buses for internal and external.

 

[MacKilRoy]

I think you're confusing the logical structure of the bus (where the Mac is at the top and everything else is underneath it) with the electrical structure of the bus (where the Mac is just another device with two scsi cables connected to it). Termination is not about SCSI as a data protocol, it's about how the electrical signals themselves travel down the wires, so it's where the wires go that matters, not what devices are talking to what. You terminate the points where the bus cables stop.



Think of the chain like this:

[internal drives] <-> [mac logic board] <-> [external drives]

The Mac is in the middle of the chain, not at one of the ends. The bus doesn't end there, it passes through. So the Mac is just a SCSI device with two ports like any other. The "ends" of the bus are the devices that only have one cable plugged in; the middles are devices with two cables plugged in.

So, essentially:

• If you only have an internal drive, the internal drive needs a terminator; the Mac can deal with the termination of the other end of the bus because it's at the end. ([internal drive] <-> [mac logic board]).
  
  
• If you only have an external drive, the external drive needs a terminator, the Mac can deal with the other end ([mac logic board] <-> [external drive])
  
  
• If you have both, you need to terminate both ends; the Mac is no longer at one of the ends of the bus, so it can't terminate anything ([internal drive] <-> [mac logic board] <-> [external drive])
  
  
• If you have more than one on the internal or the external bus, you terminate the last one on each end.

This is literally about where the cables stop. Nothing more, nothing less. If the cable doesn't stop, adding termination there won't help and may cause harm.

And obviously this doesn't apply in quite the same way for Macs that have two distinct SCSI buses for internal and external.

Thats a very detailed explanation. I think there are a few exceptions for whether internal needs termination. For example, I have experienced the odd machine like an LC where external SCSI misbehaves if no internal drive is connected (ie: no terminator internally). The misbehaving disappears if I add a small cable end and a terminator. Seemingly to indicate the SCSI bus on some Mac’s might need either a terminated drive installed, or at minimum a terminator block internally. This is on a fully recapped working board.

I have yet to figure out which Mac’s require this and which ones don’t.

 

[cheesestraws]

Seemingly to indicate the SCSI bus on some Mac’s might need either a terminated drive installed, or at minimum a terminator block internally. This is on a fully recapped working board.

I believe you're right here, but I can't remember which Macs are affected. I believe I saw a list once, but I can't remember where :-(.

 

[GerrySch]

I'd like to follow up on this as one aspect of the SCSI bus on an SE/30 that has not been clarified yet. I have a BlueSCSI internal console emulator and I can have up to seven (0-6) emulated SCSI devices (HDs, CDs, etc.). In this scenario, the internal MoBo SCSI connector is terminating one end of the bus and the BlueSCSI is terminating the other end (only one internal physical device on the bus). If I remove BlueSCSI and hook up an external SCSI HD with termination, again, the internal MoBo SCSI connector is terminating one end of the bus and the external SCSI HD is terminating the other end (again, only one external physical device on the bus). So far, so good.

Now, I re-hook up the internal BlueSCSI emulator, with its termination, and leave the external SCSI HD connected and terminating the other end of the SCSI bus. So, in this scenario, the internal MoBo SCSI connector is in the 'middle' of the bus. Is the MoBo smart enough to "turn off" that termination to have the SCSI bus work normally?

I'm restricting my question to the SE/30 as other Macs handled this differently and I don't want to confuse things any further.

Thank you,
Gerry

 

[Iesca]

As a reference for people looking for how to setup a Mac Plus SCSI chain, here's a screenshot from the manual with helpful diagrams.

The long and the short of it is, the Mac Plus doesn't have internal termination, and in general behaves a little oddly compared to other SCSI-capable Macs because of its lack of internal termination. (It is possible to add a diode at position CR1 of the logic board in order to add internal termination to a Mac Plus, a 1N4001 according to the schematic, but naturally the manual doesn't address that, so the following assumes that the diode position at CR1 is unpopulated.)

Simply put:

• If attaching a single SCSI device to a Mac Plus, attach a SCSI terminator (passthrough or not) to the second, unused port of the device.
• If attaching two or more devices to a Mac Plus, attach a pass-through SCSI terminator to the 1st device in-between the device and the SCSI cable that connects to the Plus, and then terminate the unused port of the 2nd/last device in the chain as normal.

The general assumption with SCSI is that it is always necessary to terminate both ends of the chain, but with the Mac Plus, that first pass-through terminator is not necessary when only connecting a single device. (Honestly, I don't know why. Perhaps SCSI voodoo?) However, it is absolutely required if connecting 2 or more devices, as that first terminator block functions, essentially, as the "internal" termination for the Mac Plus's SCSI controller. (Any devices in between the 1st and last devices should of course be left un-terminated.)

 

[Realitystorm]

I have some info on my site that covers how various Macintosh systems handle termination https://www.savagetaylor.com/about-us/macintosh-scsi-hard-drive-replacements/#what_is_scsi

 

[GerrySch]

I skimmed your link and even though the SE/30 is not mentioned (only the Mac SE) I assume (I hate to assume) the SE/30 and SE do the same thing SCSI termination wise.

----------------------------

For Macintosh Systems with only external SCSI devices connected (you can not install an internal SCSI device, or you have removed them all) you may need to provide external termination at the beginning and end of your external SCSI chain: This is true for the Macintosh Plus, SE, Quadra 700 and possibly other systems. I strongly suggest that if your system supports internal SCSI devices that you have at least one installed (and then follow the next part of this guide).

[mac logic board] <-> [terminator] + [2 or more external devices] + [terminator]

-------------------------------------

There is only one internal BlueSCSI desktop emulator in my SE/30 with its internal termination turned on. So I don't need a pass-through terminator at the beginning of the external SCSI bus, only a terminator at the end of the external SCSI bus. I usually don't have anything connected to the external 25-pin SCSI connector but that technically means that the SCSI bus is terminated only at one end but the bus cable is short, so it doesn't cause any specific issues with data reflections on the bus. I could plug a terminator at the back of my Mac SE/30 to be correct but it's not necessary.

Am I understanding this correctly?

Also, some simple questions. What's the boot order if I add a Floppy EMU to the external floppy connector? My internal 1.44 floppy drive works and I hooked an external Apple Superdrive which works as well. What's the boot order with an internal BlueSCSI?

Finally, how do I build a boot floppy for the SE/30? Do I need separate software or will the O/S do it for me? I know a dumb question.

Thank you,
Gerry

 

[Realitystorm]

I usually don't have anything connected to the external 25-pin SCSI connector but that technically means that the SCSI bus is terminated only at one end but the bus cable is short, so it doesn't cause any specific issues with data reflections on the bus. I could plug a terminator at the back of my Mac SE/30 to be correct but it's not necessary.

Am I understanding this correctly?

You are understanding it correctly for the SE/30 with only an internal device attached. The internal device needs to be terminated, but you do not need to have a terminator on the external connection.

 

[Realitystorm]

Also, some simple questions. What's the boot order if I add a Floppy EMU to the external floppy connector? My internal 1.44 floppy drive works and I hooked an external Apple Superdrive which works as well. What's the boot order with an internal BlueSCSI?

Finally, how do I build a boot floppy for the SE/30? Do I need separate software or will the O/S do it for me? I know a dumb question.

For boot order, I believe it is the internal floppy first, then the external floppy, then SCSI devices in ascending SCSI ID order.
For making a boot floppy, you use the OS. You initialize the floppy and then copy the system file from the system folder on to the disk.

You can also use a modern computer with a USB floppy drive to create a bootable disk.