Floppy terminators with Gotek?

[behines]

I bought a TPI Drive
I bought a used/untested Texas Peripherals drive, to use as a second drive, with a Gotek being the first drive.

Thanks to some great help from some of the folks here, my Gotek is working now. The new physical drive is not yet working - the motor spins, but it does not otherwise respond (Error 02).

I have downloaded various Tandon disk service manuals and am prepared to work my way through them. The very first thing I did was some touch-up on the worn edge connectors - am currently waiting on my conductive ink pen ink to dry. (Procedure: Clean off the oxides with solder flux. Clean off the flux with isopropyl. Apply conductive ink.) If I'm extraordinarily lucky, it will just start working now.


I have no terminators, I think.
But meanwhile, I am wondering about terminators. I figure it would be good to get this right before going off on a deep dive of all the other potential things that could be wrong.

There are no terminators installed in the drive I bought, I *think*. (The Tandon TM100 manual shows a DIP resistor socket, which the TPI drive does not have. But it does have an unpopulated 6-pin SIP R51 right by the edge connector.)

It is my belief that the Gotek does not have terminators either. But it works fine. And the cable is pretty short, about a foot total.


Are terminators really needed?
I haven't seen a lot of discussion of them in the forum threads I've read. Maybe things just work fine most of the time without them?

If they are needed, one option would be to install them in the new TPI drive. Even though it's not right at the end of the cable, the reflections from the remaining stub should not be too severe.

If I do need to install them, what is called for? Looking at the schematic for the floppy controller in the tech ref, I see that there are a lot of lines with 150-ohm pull-ups on them. In some cases, the associated gates are open-collector types (7416). Others are just standard TTL (a 74157). But not all lines have these, so it's hard to tell from the schematic whether these are terminators or just strong pull-ups to improve rise times.



And there are a lot more 150-ohm resistors in the floppy controller than there are pins in the R51 socket. The original TM100 appears to have a 16-pin DIP terminator, so as many as 15 terminating resistors. So it doesn't seem like R51 is fulfilling the same function as the DIP terminator in the TM100.


And so
So my money is on, "They're not needed in the TRS-80, and in fact, the TPI drive doesn't even accommodate terminating resistors."

Do I have that right?


Thanks,
Brad

 

[krebizfan]

3.5" drives have permanent terminators and I think the Gotek implements the same termination scheme. If the TPI is the middle drive on the cable, it should not need terminators because the Gotek will do the termination of the cable.

 

[ldkraemer]

The TPI is a Tandy Clone of the Tandon TM-100. It uses a SIP that is 150 OHM for
the Terminator Resistor R51.

RP51 = 150 OHM SIP

1  RP1-1 N MOTOR
2  RP1-2 DIRECTION
3  RP1-3 STEP
4  RP1-4 N WRITE DATA
5  RP1-5 N WRITE GATE
6  + 5.0 VDC

I have extracted the pages for the TPI from the Model 4 Manual.



Larry

 

[behines]

The TPI is a Tandy Clone of the Tandon TM-100. It uses a SIP that is 150 OHM for
the Terminator Resistor R51.

RP51 = 150 OHM SIP

1  RP1-1 N MOTOR
2  RP1-2 DIRECTION
3  RP1-3 STEP
4  RP1-4 N WRITE DATA
5  RP1-5 N WRITE GATE
6  + 5.0 VDC

I have extracted the pages for the TPI from the Model 4 Manual.

Wow, this is super helpful stuff, thanks!!

With this info and krebizfan's comment and the floppy controller schematics in hand, I realized that I should just measure things with a meter.

I found that, indeed, there seemed to be ~150 ohms between these five lines and the +5V line.

I also found that having the Gotek drive on the cable didn't change the values much. I then measured the Gotek by itself, and it seems to provide 1K terminators. After some poking around (link), I found that this is apparently a common compromise.

That note on the Dave's Old Computers site is interesting when viewed in the context of the TRS-80 drivers. Curiously, only the internal floppy connections have 150-ohm pull-ups.


I wonder how the external drive system is supposed to work? Maybe the external drive cable has pull-ups built in at the computer end? And they were left off of the floppy controller in the interest of reducing power consumption when no external drives are connected? It would be under a watt total, not that big of a deal. Curious.

What to do?
I want the Gotek to be drive 0. So what to do?

Reading further, it looks like short cables can get away with higher resistors (link). So one option is to do nothing.

If I were to put 180-ohm resistors into RP1, the net would be 150 ohms at DC. In terms of reflections it doesn't really add up that way unless the 180 is pretty close to teh 1K (which it is), but if 1K works as a compromise, then 180+1K might work even better.

The "correct" solution would be to add a little 180-ohm terminator dongle right at the Gotek. One can imagine ways to make that work from a physical standpoint. For example, one could solder 5 pins of the sip directly to the right-angle pins of the header as they enter the board, then peel up the 6th to tie it to +5V. You could even make the connection to 5V be via a Dupont connector so you could remove the termination if desired.



My inclination
I'm inclined towards either doing nothing, or putting a 180-ohm SIP into the TPI drive.

I do note that there is plenty of room on either side of the RP51 socket, so one could purchase an 8-pin or 9-pin SIP and just cut off the unwanted legs. There seem to be options on AliExpress to buy 6-pin SIPs, but they seem to bottom out at 220 ohms, whereas the 9-pins can be had in 150 or 180 ohms.


Conclusion
I'm going to buy the 9-pin 180-ohm SIP and try plugging it into the TPI drive. Soldering the the leads of the Gotek will be my backup if I have problems and the scope shows reflections.

It seems likely that it's just not necessary to do anything at all, but it's so easy to do, so why not...

 

[lowen]

...

I wonder how the external drive system is supposed to work? ....


Conclusion
I'm going to buy the 9-pin 180-ohm SIP and try plugging it into the TPI drive. ......

The last drive in the external chain is supposed to get a terminator resistor pack; DIP on the TM-100's and SIP on the TPI and others.

As you've discovered, it's not really a 'terminator' but a pull-up for the open collector drivers; true termination is to match the characteristic line impedance of the 'transmission line' bus; to keep the VSWR on the line 1:1, eliminating the reflections off of the open circuits at either end. SCSI, for instance, has both ends of the bus terminated (once on the host bus adapter and once at the end; reference https://storage.microsemi.com/en-us/support/scsi/3940/aha-3940uwd/hw_install/scsi_termination.htm )

 

[behines]

My 180-ohm SIPs have come in and I've installed one in my drive. I now measure 75 ohms from each line to +5V, as expected.

BUT, before powering it up, I paused and ran some numbers, and it seems like 75 ohms is too much load for the 7416 drivers in the floppy controller:

The 7416 is rated for 40 mA low-level output current, so

• With a 75-ohm pull-up, the signal only gets pulled down to 2V.
• On the other hand, with just the 150-ohm pull-up that's built into the floppy controller, the chip is capable of pulling the line all the way down to GND.

Given what you said about the "terminators" really being just pull-ups anyway, I conclude that one actually must NOT install them on any internal drive. But there is nothing in the Tech Refs or Service Manuals that mentions this, that I've found. You only uncover this little nugget by studying the schematic.


It seems to me that installing the terminators in an internal drive could lead to flaky performance, or possibly even damage to the floppy controller.

Does this sound correct?

Thanks,
Brad

 

[lowen]

Hmm, it's been so long I don't remember. I'll have to pull the top off my nga Model 4 to see (need to anyway, as I have a Floppy-80 to install.....)