May 12, 2015

Terminator 'B' Box on 34-pin MFM Cable

The MightyFrame S/320 has a more unique 34-pin cable for connecting to its 3 available MFM (ST-506) drives.  

The 3 drives are labeled D0, D1, and D2.

The card-edge connector for each drive is clearly labeled, and missing very specific contacts, as follows:

D0 is absent 28, 30, & 32.

D1 is absent 26, 30, & 32.

D2 is absent 26, 28, & 32.




Furthermore, there is this very curious Terminator 'B' Box on the end of the cable, after the D2 connector.  The red arrows in the picture below show where the connector contacts are missing, D2 connector and Terminator 'B' Box pictured here:



It contains a blue 220/330 ohm resistor block: 10Z221/331G 

Pins 2, 4, 6, 14, 18, 24, & 34 are all connected to the center pins of the resistor block.
Pin 16 goes to one of the ends of the resistor, and the other end of the resistor block is grounded.

David Gesswein, creator of the MFM Emulator, says:

Looks like the standard 220/330 terminator. 221 is 22 followed by 1 zero.

Each middle pin has a 220 to one end and a 330 to the other end the
resistance from ground to +5 effects the reading. Your measurement
across the middle pins is clearer. Between the pins to one end will
be two 220 ohm resistors in series and two 330 ohm resistors in series to the
other end. The parallel resistance of 440 and 660 ohms is 264 ohms which
matches what you measured.
See 104 circuit
http://www.pdp8online.com/mfm/board/datasheets/Bourns_4608X.pdf

And about the "box" itself:

Looks like its terminating Head 3,2, write, head 0,1, step, dir in. It
appears the select lines aren't terminated. I don't quite understand
why the select lines aren't terminated. I wonder if they are using a
totem pole output driver or termination on the controller board? Can
you see what is connected to pins 26, 28, 30, and 32 on the controller side?

The only way to tell would to be to use a meter to find where the pins
are connected to. You could measure the
select lines with a volt meter with no drive connected. They should be
at least 2.7 volts. To check drive I would use a clip lead to the
meter + lead to a 1k resistor clipped to ground. If the voltage is still
>= 2.7 volts then it has a pullup or is driven by a totem pole output.

Then the drives installed or my emulator shouldn't have the terminating
resistor installed.

I looked at the ST-506 schematic and the drive has the jumpers which
choose which select line to look at that then go to hard wired 220/330 ohm
resistors. 

That answers my question on why select isn't terminated in
the cable. Drives always terminate the select line they are using. 

Without some modification my emulator isn't going to work well with your terminator
cable since either lines will be double terminated or selects won't be
terminated.  You can cut off pins on the terminator resistors to only leave
gnd, 5V, and the select lines the emulator is using to make it work. If you
have the termination on the emulator you will get double termination on the
select line a second drive is using. Depending on the driver strength
that may or may not work. Also cutting terminator pins will fix.
Two emulators should work with one terminated and the other not.

[testing still ongoing, more added soon] -ajp

I now have a collection of pictures from Michael Herzog's MightyFrame in Austria Here is the 34-pin cable from that machine 

A1A6P1 = D0 Controller line 34-pin (do not use with emulator)

A1A7P1 = D1 Controller line 34-pin (do not use with emulator)

A1A6P2 = D0 Data line 20-pin (OK to use with emulator)

A1A7P2 = D1 Data line 20-pin (cannot use with emulator: plug won't fit on emulator) 



What model is this MightyFrame?  http://bit.ly/1IXXcu8
S/120 - S/221 - S/222