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SCSI termination and internal/external drives

PacificState

Well-known member
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

Well-known member
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

Well-known member
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

New member
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

Well-known member
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

Well-known member
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

Well-known member
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.
 
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MacKilRoy

Well-known member
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

Well-known member
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 :-(.
 
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