FDC on TI-99/4a

[patrickmcmichael]

Having been out of the TI-99/4A game for years, I realized how dusty my memory had gotten on the various FDC and implied capacity options. Trying to piece together info from (http://www.unige.ch/medecine/nouspikel/ti99/disks.htm#Tracks%20&%20sectors) and (https://en.wikipedia.org/wiki/Floppy_disk), I landed on the two tables below. For those far more informed than I on this, please enhance/correct.

 

 

 

[+mizapf]

For the TI, sector sizes are always 256 bytes. That means that for a 3.5" HD drive, we have 5760 sectors - if we had a suitable controller. The HFDC, for instance, could be able to handle it, but its data separator must be replaced.

The Myarc HFDC has a HDC9234 controller chip (from SMC). The Corcomp controller has a WD2793 or a WD17** chip (don't remember); the Myarc DDCC-1 has a WD1770. Note that the 1771 is the only one called "FD".

[+9640News]

I'm not sure, but wasn't there an issue with a 3.5" HD (1.44 MB) floppy DSR issue with the Myarc HFDC on a TI-99/4A? On the Geneve, the Geneve's master DSR part of the operating system had no issue. However, to use the 36 sectors per track, besides the data separator being replaced, the 8K ram chip had to also be replaced with a 32K chip in order to format the disk properly.

 

This was because the controller chip format command as I recall laid out the complete track on the memory chip before the write command was initiated. For others that are a little less informed, that is more than the 36 sectors x 256 bytes/sector, plus all the in-between sector information that was needed. It just does not fit within 8K.

 

Beery

[RickyDean]

The Corcomp controller has a WD2793 or a WD17** chip (don't remember);

It uses both, two different versions.

Edited April 3, 2019 by RickyDean

[+mizapf]

It uses both, two different versions.

 

But not on the same board. You mean there are two versions of the disk controller card.

[RickyDean]

 

But not on the same board. You mean there are two versions of the disk controller card.

Yes if you look at the other thread where we discuss the PAL/GAL on CC FDC's Helocast provides the schematics for the 1773 and I have the 2793 version, other wise they look pretty much the same. http://atariage.com/forums/topic/288694-wanted-replacement-pal-chip-corcomp-fdc-1985-rev-a-u12/ #17.

There was at least one other earlier rendition, the card is shaped more like a rectangle. More like a TI controller.

[+mizapf]

Do we already have the schematics for the CC FDC? (Not the PCB drawing; this is already available and shown in the mentioned thread.)

[patrickmcmichael]

For the TI, sector sizes are always 256 bytes. That means that for a 3.5" HD drive, we have 5760 sectors - if we had a suitable controller. The HFDC, for instance, could be able to handle it, but its data separator must be replaced.

 

The Myarc HFDC has a HDC9234 controller chip (from SMC). The Corcomp controller has a WD2793 or a WD17** chip (don't remember); the Myarc DDCC-1 has a WD1770. Note that the 1771 is the only one called "FD".

 

 

From that wiki page, I was going by this: For example, the HD format of ​3 1⁄2-inch floppy disks uses 512 bytes per sector, 18 sectors per track, 80 tracks per side and two sides, for a total of 1,474,560 bytes per disk. But if the TI is always limited to 256 bytes per track (TI computer, or TI controller, were you saying?), then to get to the 720/1.44 for the 3 1/2" you'd have to be 36 sectors per track, 80 tracks. (If double density is 18 vs 9 sectors per track, then what some called QD would make sense for 36 sectors per track. Wonder why that article implied that it was 512 bytes per sector, but still with 80 tracks and 18 sectors per track.) Either way gets you to 720 SS and 1.44 DS, I suppose.

Edited April 3, 2019 by patrickmcmichael

[+mizapf]

Floppy disk formats on the PC always use 512-byte sectors, those on the TI are always 256. Floppy disks are soft-sectored, you can choose between many different sector sizes from 128 to 1024 bytes; you can even switch between sizes in the same track.

 

Indeed, we have 36 sectors/track for such a format. But as I said, it is somewhat hypothetical, since we don't have controller support for HD (without modifications).

[+Ksarul]

I still have to convert the CC layout into a schematic. That is next on my project list. . .

[patrickmcmichael]

Floppy disk formats on the PC always use 512-byte sectors, those on the TI are always 256. Floppy disks are soft-sectored, you can choose between many different sector sizes from 128 to 1024 bytes; you can even switch between sizes in the same track.

 

Indeed, we have 36 sectors/track for such a format. But as I said, it is somewhat hypothetical, since we don't have controller support for HD (without modifications).

 

Makes sense. That one Myarc card I had with the upgraded chipset got me to at least 720K on the 3 1/2"...forget if it took me to 1.44 or not. If it was 80 track mod but still DD, that would explain top-out at 720. Anyhow...when I put in that 80-track EPROM upgraded TI card...does that upgrade only control # of tracks, but I'm still tapped out at Single Density of 9 sectors per track?

[patrickmcmichael]

A related question on FinalGrom99 cartridge images that want a disk...

 

I'll be running most things off of SD in FinalGrom99. However, some cartridge images there want a DSK1, so I'm assuming that should actually be a disk w those supporting files in DSK1, right? For example (from wht FG99 image):

 

MANUFACTURER. MFN. TITLE FOLDER WORKS? 32K? DISK? CONV? COMMENT

------------------------------------------------------------------------------------------------------------------------------------------

TI PHM 3041 Adventure GAMES yes yes

• For that, once I put the 80 track updated card into my P-Box, I will likely set up the Lotharek SD FB - HXC Rev F I have on order as DSK1.
• The 80 track upgraded TI FDC I ordered came w/ two 3 /12" drives, but I'm thinking I should use the Lotharek as DSK1 and a physical 3 1/2" for DSK2. (My card is set up as 80/80/40, and while theoretically I might have to run some legacy 5 1/4", I'd probably be able to get my hands on that same file and put it on SD for Lotharek, or worse case, get it on a 3 1/2" floppy.
• ON that note, while I don't anticipate using it much, I may order just one 10 pack off of amazon of 3 1/2" floppies. Almost everything I see there is HD, and my setup is still constrained to SD, despite 80 track upgrade eproms from what I can read up on. As long as I order the unformatted disks (or reformat them if preformatted) they should work even though w/ my setup I'll sadly top off at 360K (80 track, DSSD), correct? Being soft sector, I'm assuming this just means those disks could go up to HD, but no problem formatting as 80 track DSSD, right?

Edited April 4, 2019 by patrickmcmichael

[+mizapf]

The capacity depends on the density, and the FD1771 only supports single density.

[apersson850]

The CorComp controller (mine has the WD2793 controller) does support both single and double density.

There was also a program upgrade for the card, which made it's functionality better, but didn't affect the data storage itself.

[+Ksarul]

The schematic for the CC Card is now in work. I was able to finish about 20% of it today. . .

[RickyDean]

Will this cover b

 

The schematic for the CC Card is now in work. I was able to finish about 20% of it today.

Will this cover both controller chips, or will it be geared toward just the 1773? Curious....

[+Ksarul]

This is based on the teardown and layout already done--so (at least for now) the schematic will match that card (one of the 1773 variants, as there are at least two of those in addition to the 1293 variant).

[RickyDean]

This is based on the teardown and layout already done--so (at least for now) the schematic will match that card (one of the 1773 variants, as there are at least two of those in addition to the 1293 variant).

I am getting ready to tear down the CC FDCard, I just blew out, a couple of months ago, that we have already discussed in the thread about the PAL chip. I will get some good scans of the bare board and then a schematic can be drawn.

[+Ksarul]

Perfect--if I get the scans of the bare board, I can use them to update my schematics to cover that variant too. I'll just add another page to the schematic with the additional variant.

[RickyDean]

Jim, Here are pics and scans of two of the same variants of the 2793 variant of the CC FDC. The CC1 file is of my original FDc and the pics and scans are somewhat fuzzy, as at that time I didn't have equipment to properly do it. The second was done this weekend and the second fdc is better looking and I had better equipment. Let me know if this will fit the bill to create a good schematic or if I have to finish stripping the board. Thanks

CC1.zip

2nd CC.zip

[+Ksarul]

Those give me everything I would need for the 2793 variant boards. . .many thanks! I know how much hassle it is to strip one of these boards down to nothing, as I had to do the same to get a good schematic for the Myarc boards.

[RickyDean]

Well I had to desolder logic chips to find the other defective one that blew when I lost the 9901 and the PAL, turns out it was on of the last ones I took off, the 4069 chip.

 

I can now use this one to see why my first one is not accessing drives, by point to point checking, to see if it is a solder connection or a part that I replaced with a bad value. Then it will also get all the chips socketed and we'll go on our merry way.

[+Ksarul]

I do have a couple of questions about the 2793 boards now that I have had a chance to look at the pictures. I can't read the value on the chip with the white blob on the lower edge (next to the 74LS11). It looks like it might be a 74LS74, but I want to be sure. Also, the values for the various capacitors (the ones that aren't 104s) would be useful, as would the values for the variable resistors and the SIPs. On the schematic for the board with the 1773 controller, I am about 99% done with it. I just have to run through all of the changes discovered to date and make a final comparison against the layout and it will be done. Then I can start of the 2793 boards, which is why I started looking more closely at the pictures.

[+mizapf]

You mean this card? I don't see the 74LS11; but it is on the MG boards (with the 1773).

[+Ksarul]

That answered my chip placement questions (and their values). Now I just need the data for the variable resistors, SIPs, and the capacitors. Thanks, Michael! Your picture also definitively answered a couple of additional questions I had--the purpose of the jumpers by the ROM chip sockets, and the purpose of at least one of the Test Points. Any idea what the writing above the top variable resistor is? That's the signal it apparently modifies, unless I miss my guess.