SCSI TermPwr jumper

[Hodgkinson]

Hi,
I’m currently setting up a SCSI HDD for use with the Squirrel SCSI interface.

On most drives there's a jumper labelled TermPwr:

Open: TermPwr for on-board terminators provided from SCSI connector, pin 26. Drive TermPwr is not connected to SCSI connector pin 26.
Shorted: TermPwr for on-board terminators provided by the drive, and drive TermPwr is connected to SCSI connector pin 26.

At the moment I've set the jumper open in order to reduce the chance of destroying anything else that might be providing power to this rail.
For an external active terminator to work, does at least one of the drives in the chain have to provide power onto this pin? Or are external terminators self-powered?
Similarly, does the Squirrel already provide power onto this pin, or, if the last drive on the chain is internally terminated, would the TermPwr have to be turned on one of the drives in the chain?

[darksun9210]

hmmm, termpwr is a 5volt feed to drive active terminators, or a drives own self termination. there is usualy another jumpter to say "self termination on/off" as well.

for an external/internal _active_ terminator to work, it will need this power feed to operate. active terminators 99% of the time have a little light on them to show that they have power and are working.

passive terminators don't need this power feed as they are really just a bank of resitors pulled down to GND to stop cable reflections.

active terminators need power to provide a differential voltage to the signals (including noise) recieved on the data lines, effectivly cancelling them out. leading to a much cleaner signal path.

you won't blow anything up by having multiple devices supplying term power, but for the sake of a stable +5V feed, see if you can just have one device supplying this voltage. most of the time this will be the scsi controller itself, so you shouldn't have to worry about it, just which drives you have termination enabled on or not.

in the case of the squirrel, i have no idea. last time i played with one was about 1994/5. i don't see why not, but then it may not have one in order to minimize the power draw off of the pcmcia slot.

so really, its just a going to be a case of trial and error. you won't blow anything up, but with scsi, its going to require a bit of fiddling...

if you are going external, the best bet is to use Sun Microsystems Unipack scsi boxes
ebay link to a 36Gb drive in an external box
they present a 68Pin external connection, so you are going to have a little fun finding adapters going from the squirrel's 50pin centronics hook up, but the boxes themselves use intellegent active termination. they will self terminate both the high and low, or terminate only the high data lines depending on what they have connected.

i have in my scsi chain:-
ID0 4Gb drive (low) terminated
50pin internal
ID7 A500-GVP A530
25pin-50pin cable
ID6 CDRW
50pin-68pin cable
ID4 72Gb SCA drive in a sun box. auto high terminated.
68pin-68pin cable
ID3 72Gb SCA drive in a sun box auto unterminated
68pin-68pin cable
ID2 72Gb SCA drive in a sun box auto unterminated
68pin-68pin cable
ID5 DDS3 Tape Drive in a sun box. auto high terminated.
68pin-50pin cable
ID1 A1200 BlizzPPC (low) terminated

if i have the A1200 switched off, the box with the DDS3 drive in it will detect there is no further device, and auto low terminate aswell. saves a real termination headache, as i can pretty much (bar the CDRW which needs the A500 on to terminate the low data lines) only have turned on what i want to use at any time and the termination takes care of itself...

sorry for the ramble, i havn't much on this afternoon :-D

[Zac67]

Passive terminators DO NEED term power as well - they pull up to 5V/TP AND down to GND (220 / 330 Ohms). All term power feeds are secured with diodes, so nothing goes wrong if >1 device feed the line. -> Link

I usually set up the devices near the ends to provide TP. Those are the ones to terminate as well, but on external devices I use an external terminator instead (easier to see if everything is properly plugged together).

Some (rare) devices, host adapters etc. don't like other devices providing TP - I've never found a real reason for this, but the problem exists.

[Hodgkinson]

Thanks for the info!

I'll probably try the drive (Which has an internal jumperable terminator too) without the TP set, and I'll see if the LED on the active terminator that I have lights up or not. If not, I’ll set the TP jumper and try again.

Cheers,
Hodgkinson.

[Hodgkinson]

It would seem that the SquirrelSCSI does in fact provide its own termpwr rail output. With the termpwr jumper on the HDD open (Disabled) and an external terminator plugged into the drive, the terminator will only light up and the drive will only spin up when the A1200 is turned on.
With the drives' termpwr jumper closed (Enabled), the terminator will light up and the drive spin up irrespective of whether the A1200 is connected or not.

Related question: Suppose I upgrade from the SquirrelSCSI, to, say a Blizzard SCSI kit for my 1230IV, would I still be able to access the drives and their associated data?

Thanks,
Hodgkinson.

[rkauer]

 Since the Squirrel is RDB compatible, I suppose you're OK.

[Hodgkinson]

Thanks  ;-)