🚀 go-pugleaf

hawk@pnet01.cts.com (John Anderson) in <6630@crash.cts.com> asks:

	Does anybody know what the external SCSI connector line 25 "TPWR" is
	supposed to connect to?  The other lines are esasy to figure what they
	connect to when going to a 50 pin connector, but line 25 has me
	stumped.  What does it connect to and what is it for.  Thanks for any
	help.

"TPWR" stands for Terminator Power, and is the +5 VDC point to which the
termination resistor packs are pulled up.  And the pin is 26 (NOT 25!)

At each end of a SCSI bus, and ONLY at each end (for a total of two (2)), the
bus needs to be "terminated" with pull-ups and pull-downs.  For each data and
control signal, the termination at each end of the bus is:

	+5						+5
	V						V
	|						|
	X						X
	X 220 Ohms					X 220 Ohms
	X						X
	|						|
	+----------- ....(signal/data line).... --------+
	|						|
	X						X
	X 330 Ohms					X 330 Ohms
	X						X
      __|___					      __|___
     /  /  /					     /  /  /

Usually, a SCSI host adapter (e.g. Supra, GVP, A2091, etc.) will have the
termination on-board, and SCSI peripherals are shipped each with its own
terminator resistor pack(s) which need to be removed except for the LAST
device daisy-chained on the bus.  What I do is remove all devices' packs
and use an external terminator that plugs directly onto one of the two
50-pin Centronics-style connectors on an external "shoebox" case; these
external terminators are made by numerous companies and will cost approx.
$10 if you buy smartly or $50 if you buy stupidly (Apple accessories).

Now, the problem is to get the +5 VDC to the terminator(s) whereever they are.

For the SCSI host adapter, power to its terminator is usually supplied and
kept on-board.

For SCSI peripherals, I've seen many permutations:

	(1) some supply +5 to their on-board terminators,
	(2) some supply a jumpered +5 to their on-board terminators,
	(3) some supply +5 through a diode to their on-board terminators (OBT),
	(4) some supply a jumpered +5 through a diode to their OBT,
	(5) some do (2) and supply +5 to SCSI pin 26
	(6) some do (2) and supply a jumpered +5 to SCSI pin 26,
	(7) etc etc etc etc etc

Point being, you NEED the specific peripheral's OEM or installation manual to
understand how THAT peripheral handles pin 26.  Some manufacturers do it
"correctly" and some don't.  It's my opinion that a device with both a
diode-fed +5 and jumper selections is the safest; just imagine what happens
when multiple devices all attempt to assert their +5 onto the bus.

What I do (and I run a single-ended SCSI bus to the full limit of the specs:
20 feet) is remove ALL terminators and jumpers from all peripherals, have my
host adapter supply the +5 VDC on pin 26, and have an external terminator pack
at the last device.  I showed a picture of this setup at a recent FAUG meeting
when I gave a mini-talk about hard disks; this is the same setup I described
earlier in this newgroup as staying up 6+ months at a time, etc etc etc and
on which I have a LOT of SCSI peripherals.

Enclosed is an extract of some material I posted back in 1988 and 1989; hope
it proves useful.

Thad Floryan [ thad@cup.portal.com (OR) ..!sun!portal!cup.portal.com!thad ]

-------------------- begin enclosed material --------------------


   Perhaps now is the time to briefly describe a SCSI bus.  Consider the
   following (rough) illustration of a fully-configured (and loaded!) bus:

                                        Interface   Devices whose LUNs are:
   +----------++-----------+
   |   Your   || SCSI host |   SCSI      +-----+     +---+           +---+
   | Computer ||  Adaptor  |==========::=|SID=6|-----| 0 |--- ... ---| 7 |
   | (Amiga!) ||SID=7,LUN=0|   BUS    || +-----+     +---+           +---+
   +----------++-----------+          ||
                                      || +-----+     +---+           +---+
                                      ::=|SID=5|-----| 0 |--- ... ---| 7 |
                                      || +-----+     +---+           +---+
   where:                             ..
                                      ..
   SID = SCSI ID [0..7]               ..
   LUN = Local Unit Number [0..7]     || +-----+     +---+           +---+
                                      ::=|SID=0|-----| 0 |--- ... ---| 7 |
                                      || +-----+     +---+           +---+
                                      ||
                                   -/\/\/\-
                                Bus Termination

   Given the proper interface at each SCSI ID, one could place up to, say, 8
   disk drives on it, and with 7 interfaces, one could have, say, 56 disk
   drives on one's bus.  This is what the SCSI bus and its specs permit.

   [...]

3) Another set of potential problems concerns an improperly terminated SCSI bus
,
   miswired cabling, and/or mis- or non-use of the SCSI terminator power line
   on the bus.  SCSI bus wire 26 has been perverted by many manufacturers and
   users, and others have "done" non-standard things with other "undefined" SCS
I
   bus wires.

   In a previous posting, I've detailed the SCSI bus lines that one should be
   SURE are not connected (improperly).  I think this was also reprinted in one
   of the Amiga-related magazines, but I haven't seen it.

   In any event, among the mis-wiring offenders I've seen are included Supra,
   Seagate, Apple, etc.   And a lot of "Apple" 3rd party disk subsystem assembl
y
   houses really screw the bus by having too long a "stub" off the bus (spec is
   10cm MAX, and I've seen up to 40cm; sheesh!).

4) And a BIG problem with using Maxtor drives is the use of an inadequate
   power supply.  These mongo drives require tremendous startup-surge capabilit
y
   AND very clean power during operation.  Doug should look at both the +5 and
   +12 VDC lines on a 'scope while powering-up and running the drive.

   [...]

-------------------- end enclosed material --------------------