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TM-848 8" Floppy Drive Error Codes

pski

pski

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The Tandon TM-848 8" floppy drive will display various self-test error codes on startup by flashing its LED in a particular sequence. The User's Manual describes these on page 1-3. However, I have a drive that is flashing in an undocumented sequence: 3 flashes, then 1. Anyone here know what that sequence means?

I should also add that it makes a 1 second grunting noise at startup too.

Here's a copy of the manual I'm using: https://dl.dropboxusercontent.com/u/3308843/TRS-80/Tandon_TM848_User_Manual_1983.pdf
 
pski said:
The Tandon TM-848 8" floppy drive will display various self-test error codes on startup by flashing its LED in a particular sequence. The User's Manual describes these on page 1-3. However, I have a drive that is flashing in an undocumented sequence: 3 flashes, then 1. Anyone here know what that sequence means?...
Click to expand...

I never ran across that blink code, sorry. But, if you were so inclined to do so, you could go through the firmware and reverse engineer it: http://bitsavers.informatik.uni-stuttgart.de/pdf/tandon/firmware/Tandon_TM848-E_8748.hex

It's 8748 code, and so it wouldn't be super hard to reverse-engineer. You do have the 'E' drive, right? I couldn't find a blink code for the non-E TM-848-2 in it's service manual (http://bitsavers.informatik.uni-stuttgart.de/pdf/tandon/TM848_service_1982.pdf ).
 
lowen said:
I never ran across that blink code, sorry. But, if you were so inclined to do so, you could go through the firmware and reverse engineer it: http://bitsavers.informatik.uni-stuttgart.de/pdf/tandon/firmware/Tandon_TM848-E_8748.hex
Click to expand...

Sounds like an interesting idea.

lowen said:
It's 8748 code, and so it wouldn't be super hard to reverse-engineer. You do have the 'E' drive, right? I couldn't find a blink code for the non-E TM-848-2 in it's service manual (http://bitsavers.informatik.uni-stuttgart.de/pdf/tandon/TM848_service_1982.pdf ).
Click to expand...

Yes, its an 'E'. The 'E' is a more intelligent drive compared to its predecessors with a microprocessor and hence the firmware option. The older dumb non-'E' does not have the blink codes. This is the first 'E' I've had to troubleshoot. Seems like it should be a more reliable design. I see it has a diagnostic mode too. I'm going to play around with that to see if it gives me a clue.
 
Well, no matter what I do, diagnostics mode on or off, 50pin cable connected or not, I still get the same 3 flash 1 flash. Motor spins constantly too. Guess I'll learn a little 8748. :)
 
Grunting sound might be indicating that it can't detect
where track 0 is.
You might put a meter on the sensor ( switch or opto transistor ) and
see if it is detecting the track 0 position.
Dwight
 
I've only got the non-E versions, but here's a guess.

Let's assume that the drive tests things in a certain order. What would the first test be? Perhaps it's that the spindle motor is spinning at the right speed? Are there any sensors or tach signals that you could monitor that might indicate that?
 
I think you guys are on to something. It appears that self test is supposed to step out from track 0 and then go back to track 0 and stop. It does step out and seems to groan when it hits the stop at the other end but then never goes back to track 0. TP3 is always showing low which is the track 0 signal. Maybe I have a bad track 0 sensor or wiring.
 
pski said:
The Tandon TM-848 8" floppy drive will display various self-test error codes on startup by flashing its LED in a particular sequence. The User's Manual describes these on page 1-3. However, I have a drive that is flashing in an undocumented sequence: 3 flashes, then 1. Anyone here know what that sequence means?

I should also add that it makes a 1 second grunting noise at startup too.

Here's a copy of the manual I'm using: https://dl.dropboxusercontent.com/u/3308843/TRS-80/Tandon_TM848_User_Manual_1983.pdf
Click to expand...

That grunt on start, sounds like my better half when she gets worken up by one of our inconsiderate neighbour's starting a chain saw at 6am. Mind you her grunt last way longer than a second.

Being serious now, does it make the same flash sequence and grunt if you have the door open and no disk in drive ?

I am making the assumption your original comment was meant to indicate what the drives does the instant you closed the door, after you had inserted a disk, then allowing it to start up. ?
 
inotarobot said:
Being serious now, does it make the same flash sequence and grunt if you have the door open and no disk in drive ?
Click to expand...

The TM-848-E will be unresponsive to door or disk presence until it completes the self tests.

Ok, i shorted the track 0 sensor and now the self test completes an the head shoots back to track 0. I think its a bad track 0 sensor.

Anyone know where I could get a replacement? Are they still making these?

Thanks!
 
That's a Honeywell part (HOAxxxx), but my catalogs don't carry it. Absent finding an old operating one, you may want to consider something like an HOA1887:

RF321930-01.jpg
 
When a drive is powered up, normally it will step IN towards the spindle a specific number of steps (after the Track 0 Sensor turns off) off the Track 0 indicator, and then steps OUT (towards the outer edge of the disk) until it get a Track 0 indication from the Track 0 Sensor to calibrate Track 0.

If the drive is stepping in on power up, and continues to step In towards the spindle until it hits the mechanical stop at the high Track, and stops stepping in when the Track 0 Sensor is disconnected, and starts stepping out, then the Track 0 Sensor is bad. Either the sender side is not sending, or the Receiver side is not receiving. The drive thinks the heads are on Track 0 no matter how many times it steps the heads towards the inside (which is why it keeps stepping in). If you disable the Track 0 Sensor, I would think that the Stepper motor would first step IN, and then step OUT and continue to try to step OUT even after the head assembly hits the mechanical stop.

Many of the parts on 8" floppy drives were made specifically for a particular Brand and Model of Drive. I remember seeing new optical sensors for Shugart and Siemens 8" floppy drives for years in bins in surplus electronics stores. I never saw many parts for Tandon 8" floppy drives in surplus stores. About the only way that you are going to get an exact replacement is going to be taking it off a donor Tandon 848E drive if you can find one or have one (I don't remember if the belt drive models used the same Track 0 Sensor). The TM848E drives are much more scarce than the older belt drive TM848 drives. Those sensors are some of the only relatively reliable parts that were installed on Tandon drives. The odds are, that if you can find a 848E drive to use for parts, and if the sensor is on it, the sensor will be good.
 
Hi Pete,

It probably will Not happen till Monday or Tuesday next week, but I will swing by Truscott Electronics here in Croydon, a suburb of Melbourne Australia and see what sensors they have in their misc bins. 5% chance they may even have an old drive about.

I also wonder if the track zero sensor from a 5 1/4" drive may do at a pinch. Certainly way more of these smaller drives kicking about to get and rat for bits.

Will let you know.
regards
David
 
Last edited:
Most any of the 4 terminal opto sensor should work. It is more an issue
of the mounting. Worst comes to worst, some hot glue would make
a temporary fix.
Dwight
 
I'm doing some drive service for a company that has a number of non-working Mitsubishi M2896-63 and Tandon TM848 drives. Not sure what model TM848s they are (could be belt drives or Es). They sell and/or support an older product that uses 1/2 height dsdd floppy drives. Most of those machines have been retired, but they still have a few out there, and do data transfers for their clients. Apparently they ran out of working drives, and sent the quartet of drives to me to get a couple of runners.

The Mitsubishi drives that I have gotten have been in rough mechanical shape (out of four drives that arrived, I found four bad logic boards, and three bad sets of heads), which means that it was more economical to swap drives, than to fix and return the drives. I'm going to asking my contact at the Company if they are interested in selling the Tandons and whatever Mitsubishis they have left outright.

Would you be interested in a non-working Tandon TM848E (if that is what they have) to be used for parts if I can make a deal for the Tandons? What would it be worth to you? Previously I wrote that you need to get a replacement sensor from another TM848E, because the sensors from a different drive might not fit (or in the case of TM848-1 or TM848-2) the sensors might have the wrong length of wire on them. From what I remember of the TM848-1 and -2 and the TM848E, the wires on the sensors are different lengths, and they cannot just be swapped from one model to the other.
 
The HOA1887 arrived this week and I jumped back into this issue tonight. The HOA1887 is a perfect replacement for the Track 0 sensor on the TM-848-E. However, it did not fix my problem. So I investigated further.

It turns out the diode in the Track 0 sensor is in series with the diode in the Write Protect sensor and that diode was bad!

Screen Shot 2016-09-02 at 11.52.43 PM.jpg

In hindsight, I should have pulled the Track 0 sensor and tested it in isolation. Lucky for me both sensors are identical so I replaced the Write Protect sensor with the original Track 0 sensor and the drive passes all self-tests now.

Thanks to everyone for your help!
 
pski said:
The HOA1887 arrived this week and I jumped back into this issue tonight. The HOA1887 is a perfect replacement for the Track 0 sensor on the TM-848-E. However, it did not fix my problem. So I investigated further.

It turns out the diode in the Track 0 sensor is in series with the diode in the Write Protect sensor and that diode was bad!

View attachment 32936

In hindsight, I should have pulled the Track 0 sensor and tested it in isolation. Lucky for me both sensors are identical so I replaced the Write Protect sensor with the original Track 0 sensor and the drive passes all self-tests now.

Thanks to everyone for your help!
Click to expand...

I don't remember ever seeing sensors wired that way before. Normally, sensors run in parallel, isolated from each other. That makes troubleshooting much easier. Ever try to find the burnt out Xmas light on a string that's wired in series? I would not have thought to test the sensor off the drive. When a sensor doesn't work on the drive, there's no reason to think it would work off the drive (I've never removed a sensor and replaced it with the replacement sensor not fixing the problem). Normally the wires from the sensors run directly to a connector on the logic board, and to logic on the logic board, not from emitter to emitter (on a another sensor) that wired in series.
 
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