Re: Tandon TM100 Termination Resistor Network

Mike Y wrote:
I may be wrong (heck, I can't remember lunch yesterday let alone this
stuff from 25 years ago!) but the resistor packs were not needed when
the controller boards had short fixed runs of cable. Something you
could do with the drives in the case when there wasn't a common
'daisy chain' that hung outside the case. Just put the pull-up on the
FDC card and it was fine.

The move to split the 'daisy chain' was probably somewhat influenced
by the Model II which originally kept everything on one cable, internal
as well as external, and had that 'terminator plug' which was actually
a wire-loopback. Customers had a LOT of problems with that. Just
forget to turn on the expansion disk bay and boot the machine... When
attached, the bay provided the pull-ups. However if it was 'off' then
the bay had 'pull-downs'. Which included the 'write' signal and as soon
as you closed the door on your main drive to boot the floppy...

When the Model II FDC card was redesigned, the floppy 'chain' was
split into 2 isolated chains, and configurable as either 1-3 or 2-2,
depending on whether the card was to be used in a Model II or 16.

Mike


"Ian Mavric" <ianmav@xxxxxxxxxxxxxxx> wrote in message
news:f4r5oo$2601$1@xxxxxxxxxxxxxxxxxxxxxxx

Neither the Model III or 4 have internal drives that are terminated. For
whatever reason its not needed (Frank Durda IV?), only the external drives
need termination.

Frank - is there any benefit from adding termination to the internal drive

0

in a Mod 3 or 4?

Ian.


"eugene" <trs80@xxxxxxxxxxxxxx> wrote in message
news:1181288169.072533.93560@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

I just rescued a Model 4 in sad shape from an electronics recycler.
It won't boot from drive 0 and the motor is acting really strange. I
pulled both drives out and neither one of them is terminated! I put
in a Teac FD-55BV just for testing and it boots. So the machine is
working.

My question is this, I know that one of the lines that is terminated
by the resistor network is the Drive Motor Enable. Since it isn't
being terminated this could explain the strange motor behaviour. I
don't have a termination resistor network. I'm assuming that it's
just a 150ohm network wired straight through? That is, 150ohms
between pins 1 and 16, 2 and 15, etc. I couldn't find specifications
for the resistor network in any of the TM100 documentation other than
a web site which claimed it to be 150ohms. That seems small to me.

Does anyone know the specifications on the termination network?




The floppycontroller card has 150ohm pullup to 5V on the upper internal connector j1 outputs on pins 20, 18, 24, 22, 32 and 16 this is the terminator and works when the cable is short. It is R3 and R2 where R3 is a resistor network of 5 resistors. On the bottom external connector j4 there is no such resistors so there the terminator needs to be at one of the drives.

So the Model III/4 internal drives should not be terminated. At least not with any low ohm terminator. I have 3.5" drives as :0 and :1 and they have high ohm terminators, and this also works fine.

If the drives are close to each other it doesn't matter much which one has the terminator.

In principle the terminator should be at the end of the cable, furthest away from the driver, regardless of the cable length.

But it even works as they have made it in an IBM PC where every drive is terminated (max 2 there). Compared to that the model III/4 way to do it is nice. :-)

--
Knut
(delete 'nogarbage.' for email)
.

Relevant Pages

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