July 11, 2015

Wangtek 5099 Tape Drive Main Board #30509 "Tap-In"

In an effort to eventually build a device that reads raw magnetic flux transitions on all QIC-11 and QIC-24 tape formats (plus a few others, hopefully), this page is dedicated to the analysis of the technical design and circuitry of the Wangtek 5099 Tape Drive with Main Board Assembly #30509.

The most comprehensive manual for this tape drive is the 

OEM MANUAL - SERIES 5099EN QIC-36 INTERFACE - 1989

Before we begin, I'll point out that the main card-edge connector on this board seems to contain the connections necessary to at least read "data pulses", which would be the interpretation of the magnetic flux transitions on the tape.  These are standard pins on a QIC-36 interface, which seems to be what the main connector on this board provides
Pin 2 - Go
Pin 4 - Reverse
Pin 26 - Data Read Pulse
Pin 38 - Tachometer


The overview for the 30509 board begins on PDF page 64

The schematics for all the boards begins on PDF page 112

FRONT
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BACK
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MOTOR CONTROLLER
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---------- Forwarded message ----------
From: Tom Trebisky 
Date: Mon, Aug 17, 2015 at 10:20 PM
Subject: Re: TTL termination
To: Convergent MightyFrame


Home now and really tired for some reason, which is odd since I woke up feeling unusually well rested this morning.  Anyway, I won't dive into the Wangtek manual and specific signals just now, but intend to in the morning.

However, some general comments about terminators.

1) First off you are correct about just driving them with a TTL signal, that should work fine.
Some systems expect to be driven with an "open collector", which boils down to either connecting
them to ground, or not with a switch (which is actually a transistor).  I will find out tomorrow if some
of these signals expect open collector drive, but I am betting not since they don't usually use the
220/330 terminator, but rather just a "pullup" to 5 volts.

2) A TTL signal in the good old days was 5 volts high, 0 volts low.  Actually it is more complex than that,
but that certainly works.

3) So why the terminator you should be asking.

3a) It is absolutely needed if you are doing open collector.
3b) It may be necessary in other situations, but usually is not strictly required but is "good practice".
The 220/330 biases the signal in a "happy place" when it is not being explicitly driven and helps with
noise immunity and speed.

4) If you get into SCSI, you will have to deal with only terminating things at the end of the cable.
It is a 220/330, but no need to get into SCSI as we aren't talking about that yet, eh?

[NO SCSI AT THIS TIME]

5) MFM drives also used 220/330 terminators as I remember and may have had rules similar to SCSI
if there were more than one drive on a cable.
[YES, SEE THIS PAGE]

Now let me see what good old Horowitz and Hill say and maybe I can recommend some pages in the
Art of Electronics.   Someday we may have to talk about RS-422 and differential data signals.
I am not finding anything that is worth recommending you dig it out.

I am wanting to find something more specific about TTL high and low levels.  It turns out this
depends on the specific logic family.  In there era of the Wangteks and so forth (1980's) this
was either "vanilla" TTL or "LS" series.  Here is some information from a Motorola databook
I was lucky to find handy:

Anything greater than 2.0 volts will be sensed as "high" by a TTL input.
Anything less than 0.7 volts will be sensed as "low" by a TTL input.

So you can see how the game goes.  Look at page 24 of the Wangtek manual you sent me.
Now consider a terminator with 220 going to +5 and 330 to ground.  That divides the 5 volt
supply and provides 3.0 volts to the input of some TTL sensing stage, which cleanly drives
it to a logic high without any other signal.

Note that a terminator is only used on signals coming IN to the Wangtek.

A TTL output that wants to pull low can sink enough current to pull things into the logic low
regime.  An open collector can do so nicely also.  You could get busy and calculate how much
current you would have to pull to ground to get into the logic low realm (below 0.7) but I am
too lazy and I know it works, so why bother