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TRS80 Model II drive terminator

JonB

JonB

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Joined
Jan 26, 2014
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South Herefordshire, UK
How sensitive is the FDD interface on the MII? I have tried to make an adapter for plugging a 34 way Shugart drive into the 50 way plug in the back of the machine but it will not boot with it connected. There is a header for the terminator plug thing and it is all wired properly, I checked. It doesn't work with the 34 way drive either, though I think that is a different problem. Despite that, with no 34 way drive or cable connected, it is pin to pin correct on the terminator. So, with a 50 way cable connecting the adapter PCB with the computer, and the terminator connected to the end of the adapter, it is failing to boot. I don't get it, any ideas? It's tested for continuity from the internal edge connector to the adapter to the terminator, it's all correct. There are no shorts either. So what might the problem be?

Cheers
JonB
 
ISTR that the 8" drive board jumpering for the Model II is a bit whacky (it was discussed here a couple of years back) which enables easy addition of an external drive box. I'd think that reverting to a "nornal" signal routing with plain old 150 ohms-to +5 termination would work just fine after that.
 
Hi Chuck.

Are you saying it should work? I believe the terminator is connecting each even (signal) wire via a resistor to 0v. It must be like this because the odd numbered pins are grounded (although I haven't checked them all, but if they weren't it wouldn't be a proper Shugart pinout).

(Edit, just checked the service manual, the odd pins are grounded.)

It says in the service manual "The terminator is a PC board which provides jumpering to the termination resistors on the on the computer's FDC PC board." As I said, I thought it provided a set of parallel resistors for each signal wire to ground. Have I got that wrong?

Errm, confused now! :confused:
 
Last edited:
You have to understand that the only drive-terminated signals will be drive inputs; the floppy bus architecture is basically old-style open-collector. So the last drive on the chain pulls up controller outputs and the controller pulls up controller inputs.
 
@Chuck(G)

Yes, That's the way it should be done.

@JonB

One remark: The termination resistors (or array) should be connected to the + 5 volt not to ground.

If you have a Model II with a Shugart 800/801 diskdrive and an old type controller the drive should be terminated on
the outside 50 pin connector.

If you have more drives, then on the last one.
If you take a look at the card box side of the drive and you see transparant purple wires going from top to down,
then there is special Tandy modification.
This modification redirect the drive termination to empty wires on the connector.
The termination is done by a special connector which must be plugged into the external connector.

Pin 4 connects to 40
Pin 6 connects to 38
pin 8 connects to 36
pin 10 connect to 34

HPIM0145.jpg
 
JonB/Hans01,
I'v started looking at the schematics for the 1791 FDC (U6) and the associated wiring to Connector J1. Here is what I've documented so far
starting with the 8" Floppy Drive IDC Connector, and associated wiring to Connector J1.

See attached file. View attachment 8Floppy.txt

On U6 Pin 29 TG43 (Track Greater than 43) it goes to some Pin E that is maybe... EXTRN?? It also goes to
U16 Pin 9 thru gate to U16 Pin 6, to J1 - Pin 2, which isn't documented well on Schematic as being used. Or
does it really terminate on Pin 2? Is this connection correct on the actual hardware?

Larry
 
As mentioned in earlier threads, the Model II uses a peculiar drive jumpering for the termination connector, "looping back" some signals. There's nothing to stop you from going "conventional", however. I've run 3.5" drives from the "new" controller (the one with a 34-pin header pad layout) on my model 16.
 
Hey Chuck-

I'm assuming that you mean the late model FDC. Have you soldered a 34 pin header onto the pads? And, if so, have you found that the FDC was really ready to go, only needing the header, or were any other modifications necessary?

Thanks!

Chuck(G) said:
As mentioned in earlier threads, the Model II uses a peculiar drive jumpering for the termination connector, "looping back" some signals. There's nothing to stop you from going "conventional", however. I've run 3.5" drives from the "new" controller (the one with a 34-pin header pad layout) on my model 16.
Click to expand...
 
Yes, my Model 16 FDC now sports a 34-pin header. Since I wanted to use an unmodified PC 3.5" 1.44M drive, I also jumpered READY/ true and jumpered Select 0/ to Select 1/. Most 3.5" HD drives do not have an option for READY/ output, but some do, or can be jumpered for it (e.g. Samsung SFD-321B) and most are pre-configured for DS1. I did the jumpering on the 34-pin header using wirewrap wire, so it was easily removable.

If you wanted to be letter-perfect true to an 8" drive, you'd also jumper the drive for 1.23M (i.e. 360 RPM), but I didn't bother.
 
This is very useful. I think that, rather than make another 50 -> 34 pin cable (which I apparently screwed up somehow but am really, really close) I'll just solder on a 34 pin IDC header and try out my HxC that way.

Thank you!


Chuck(G) said:
Yes, my Model 16 FDC now sports a 34-pin header. Since I wanted to use an unmodified PC 3.5" 1.44M drive, I also jumpered READY/ true and jumpered Select 0/ to Select 1/. Most 3.5" HD drives do not have an option for READY/ output, but some do, or can be jumpered for it (e.g. Samsung SFD-321B) and most are pre-configured for DS1. I did the jumpering on the 34-pin header using wirewrap wire, so it was easily removable.

If you wanted to be letter-perfect true to an 8" drive, you'd also jumper the drive for 1.23M (i.e. 360 RPM), but I didn't bother.
Click to expand...
 
Chuck,

Are you using J1 or J2 for your 34 pin header? J1 is the internal interface and J2 is the external. If using J1, are you also using the internal 8" floppy along with your 3.5" floppy?

Thanks
 
J1 on this controller:

FLOPPY_DISK_CONTROLLER.jpg


I have the 2 8" floppies (model 16) disconnected at that point.
 
Thanks for the info, guys.

I see the links on the SA800 board and they are also shown in the service manual. This drive worked unmodified when connected to my PC using that funny cable with the prototyping links I photographed a while ago. So I guess the links have no effect on the 34 pin interface. Now, my adapter board takes the 50 pins from the back of the MII and presents them to a female 50 pin socket as a pass through, so what I'm not getting is why the thing refuses to work with the terminator plugged in. I may have made an error in my wiring, it's true, but I tested all of it, from the edge connector in the MII to the output socket on the board where the terminator fits. Looks OK...

Oh, PS, I sure wish I had a FDD controller like that, Chuck!

:)

JonB
 
Well, I put the header into J1 and hooked up the HxC... No good. Seems to select ok but never reads anything or advances the track.

I've got a couple of more headers on the way so I'll outfit J2 as well.

dhoelzer said:
Wonderful. I'll report back this weekend when I'm back in range of my '80.
Click to expand...
 
Jumper the HxC, yes. Pull pin 34 to ground... errr... no? :) I'll figure out a way to slip something in there and do that. Easiest way might be a jumper wrapped around pin 33/34 on the card.

Chuck(G) said:
You did jumper the drive for DS0 and pulled pin 34 (READY/) to ground, didn't you?
Click to expand...
 
Ok, so now the answer to both questions is "Yes" and there's still nothing. I get the RA and the select light but never a track advance. Are you sure you're on J1 and not J2? I haven't traced it out but J1 is right next to the external connector while J2 is right next to the internal connector... :)

Chuck(G) said:
You did jumper the drive for DS0 and pulled pin 34 (READY/) to ground, didn't you?
Click to expand...
 
dhoelzer said:
Ok, so now the answer to both questions is "Yes" and there's still nothing. I get the RA and the select light but never a track advance. Are you sure you're on J1 and not J2? I haven't traced it out but J1 is right next to the external connector while J2 is right next to the internal connector... :)
Click to expand...

J1 is definitely on the internal interface and J2 on the external interface on the late FDC card which is pictured.
 
Weird. Well, I've already got the header coming so I'm going to install it anyway and see what happens. Thanks for your patience. :)

pski said:
J1 is definitely on the internal interface and J2 on the external interface on the late FDC card which is pictured.
Click to expand...
 
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