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patrickmcmichael

FDC on TI-99/4a

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You mean the part with the yellow dot at the top, between the 2793 and the 7604, and there the black writing on the PCB?

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Jawohl, Michael. :) That was the writing I could only partially see. The blue part with the screw is a variable resistor, and the yellow dot is there to hold the set screw in place so that the setting won't drift. The somewhat hidden text will help inform me on the signal being modified by that variable resistor. Vielen Dank!

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I will have to pull the removed sips from storage to get the values from them (had grandchildren in for Easter, so stored everything), I am not absolutely sure I ordered the correct values from Jameco, as I couldn't find info on how to read the older nomenclature and translate to the new nomenclature. But I will find the sips and take some close up pictures of them, unless someone beats me to it.... ;) :)

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The middle pin of the resistor leads to the WPW pin (33) of the WD2793.

 

WRITE PRECOMP WIDTH: An external potentiometer tied to this input controls the amount of delay in Write precompensation mode.

(from WD279X-02 Floppy Disk Formatter/Controller Family)

 

WRITE PRECOMPENSATION

When operating in Double Density mode (/DDEN=0), the WD279X has the capability of providing a user-defined precompensation value for Write Data. An external potentiometer (10K) tied to the WPW signal (pin 33) allows a setting of 100 to 300 ns from nominal.

Setting the Write precomp value is accomplished by forcing the /TEST line (pin 22) to a logic 0. A stream of pulses can then be seen on the Write Data (pin 31) line. Adjust the WPW potentiometer for the desired pulse width. This adjustment may be performed in-circuit since Write Gate (pin 30) is inactive while /TEST=0.

 

Precompensation is particularly needed for hard disks, but can be used for floppy disks, too. The closer you get to the spindle, the tighter packed the sectors are. This is because we have the same number of sectors in inner and outer tracks. For a safe recording, this difference may need to be compensated.

 

Edit: If you have GIMP, open the attached file (unpacked) and change the visibility/transparency of the layers to check.

front_PCB_xray1.zip

Edited by mizapf
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The other potentiometer obviously connects to the RPW pin of the 2793. It is a 50 KΩ poti in accordance to the 279X specs; it must be used to initially adjust the read pulse width for the internal data separator.

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I just tried to get my old TI FDC to run with my Geneve (for testing with Rompage). However, booting fails immediately: The motor is triggered, the card LED is turned on, but the Geneve immediately issues a message that it cannot boot. It does not even wait for spin-up or try to move the head. That is, the motor starts spinning, and almost simultaneously, I get a honk.

 

(Maybe the card was actually broken back in the past, and this was the reason to get a new one, not just the desire to switch to double density.)

 

@Tim: From your knowledge of Geneve boot behavior, could this mean a bad controller? I don't think the 244 drivers are bad, since the SBO 0, SBO 1 commands seem to work. The EPROM on the card is not used, I suppose.

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Possibly, or some other related logic. The Geneve should boot just fine with a Corcomp or TI controller,so long as the disk is readable, at the right density, and has the appropriate files. Troubleshooting a peripheral card with a Geneve is like trying to use the TI without a cartridge slot - not impossible but pretty close to it. ;)

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Note to myself: We should include some obscure diagnose codes for the new boot EPROM. (Cannot boot - Error #1e00593c)

 

(Well, Windows has no problems showing 8-digit hexadecimal error codes to users.)

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Jim, here are the locations and values of the sip resistors on the CC FDC. Let me know the equivalent current numbers that you know to be correct, so that I can know if I deciphered them and ordered the right parts for replacements. Color coding should be self evident. Thanks a lot.

 

Ricky

post-41593-0-27973300-1556198431_thumb.jpg

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Matt,

 

Is there any chance the TI FDC was modified for a TI-99/4A for 80 track use? If it is, the Geneve boot eprom may not be smart enough to handle the modification as it didn't exist back in the day.

 

Beery

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Is there a hardware change involved in the 80 track mod? To my understanding, the track count is a software concern (other than FM/MFM, which is hardware-related).

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I am not sure if there is or not a hardware mod. There was a modification that had circulated around the very early 90's, maybe even late 80's. I am not sure if that mod is the same one people have talked about in the past few years or not.

 

What I seemed to recall with the late 80's/early 90's mod, was that it was only good for the user as he would not be able to exchange disks with other users. Someone would have to go in and dig around in the Geneve boot eprom to see how it determines what FDC one has. I know the eprom has some low level non-DSR code for each controller. Just not sure how it detects and/or maybe if an 80 track mod confuses the boot eprom.

 

At this point, we do not even know if Matt has an 80 track TI FDC.

 

Beery

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The TI FDC 80-trk mod involves changes to the two chip DSR and allows a TIFDC to write 80-tracks to all or any of the three drives available to the TI FDC. It was originally put together by "The Great Gazoo".

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The Master DSR would happily format 80-track disks for ANY of the PEB controllers, IIRC. . .as long as the drive supported it.

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Yes, as I guessed. The track number is a physical limitation of the drive, and the only signals from the controller to the drive are STEP and DIR. As long as the controller supports the track number, the drive will move the head inwards until it hits the inner limiter, or outwards until it reaches the outer track. If I remember correctly, the 1541 of the C64 had a bug that made it pull back the head beyond track 0, creating the hammering sounds.

 

The maximum track number has a meaning when calculating the coordinates (Cylinder,Head,Sector) from the linear sector number.

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The Geneve master DSR supports 80 track with ti, cc, and myarc cards. SETDSK is the command you need to use. I do not recall if the Geneve EPROM 0.98 integrated 80 track support for one or more of the controllers. Inspecting the eprom source should reveal the answer.

 

(The Geneve 80 track/boot support probably needs its own thread before this one veers too far off course)

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