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Bootstapping Cromemco 4FDC

E

Endersending

Experienced Member
Joined
Mar 14, 2023
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119
Location
Walker, Minnesota
I am currently working on an IMSAI 8080 System with a Cromemco ZPU and 32K of static ram.
I purchased a Cromemco 4FDC with Rdos 1 on it. I have been able to access the serial console and use rdos but that is about as far as I can go. My next step is to boot CDOS with 8" floppies but I have hit multiple roadblocks. I do not currently have a CP/M or CDOS formatted disk. Rdos 1 cannot format disks. I am also learning that both my 8" drives need to be fixed/realigned. The first drive will read and write until about track 25 and starts not reading/writing correctly. The other is not responding correctly to head steps and head loading.
I have been using ImageDisk head alignment and trying to write 8" floppy images. This setup is being done with a 286 PC and a 34 to 50 pin adapter that I bought online. So I am currently at a chicken and egg problem. I cannot format a disk. I can't aligned my drives because I have no formatted disks. I have watched a Curius Marc video where they use an oscilloscope to align the heads with an alignment disk. I bought some 8" floppies with CP/M software which I figure will work to use with an oscilloscope and ImageDisk to aligned it properly.
I have also learned that none of my floppy controllers can write 128 byte sectors verified by ImageDisk's TESTFDC program. I am guessing I need a controller that will do 128 byte sector writes to create a proper disk. I have ordered a AHA-1542B scsi controller with onboard floppy controller that is listed as working with 128byte sectors. I guess I need to wait until that arrives with some formatted CP/M disks.
I have been reading through the Glitch works tutorial on transfering CDOS over serial. This doesn't work with rdos 1. I keep getting a com timeout error trying to transfer the image. So my questions will start with rdos 1 and should I burn a new 2708 eeprom with rdos 2.52?
Since I am using a Cromemco 4FDC and I also ordered a TUART board for serial, I figure my best path is to use CDOS as the operating system for this machine. Is this a wise choice?
Once the machine is running with an OS and I can always try different CP/Ms.
Advice on moving forward would be great. I have my 8" floppy drives (single sided, single density, soft sector) I would like to use, although I might start with 5.25 360K floppies. I have been confused with which floppy drives I should be using. Most of the CP/M images I have found will say 8" or 5" but not density. Like I have said I am using ImageDisk and I am not sure if the images I am writing are the correct density. Here is a link to Dave's Old Computers where I am planning on using the Cromemco Rdos package to boot the system. In the zip file there are two disk images, but I am unsure of the density. Should I be using double density 8" drives? Can the 4FDC mix density and size floppies? can I run 2 a single density 8" with 2 doulble density 5.25" drives? From the way I understand it, the controller supports one size or the other, not both at the same time. Please correct me if I am wrong.
It is looking like this will be an IMSAI case with Cromemco Guts and software. Any advice to the above ramblings would be greatly appreciated.
I will post back as soon as the floppy controller and formatted disks show up and I can get some floppy drives writing disks with ImageDisk.
Thank you for the VCF Site. iI's great to have a connection to people with knowledge about this stuff.
 
An IMSAI case with Cromemco guts is...well very cool. You've basically recreated a Z-1. Now, onto a few items. Did you just buy that 4FDC off eBay from the seller in the Pacific Northwest? I see that listing is gone. That board was missing some parts including the FD1771 floppy disk controller chip. I hope you got a different one!

The 4FDC is a single density disk controller. If you want double density, then a 16FDC or 64FDC is the correct type, The 4FDC and 16FDC were designed for Persci disk drives. They use a slightly different pinout than Shugart bus drives, which may be causing some problems as well. Either the cable or board mods can handle the slight differences.

Updating the RDOS is a fine idea. However, I don't know what happens if you use an RDOS that expects a 16FDC or 64FDC since they have double density support. I've never done that particular update to my own 4FDC since I don't currently use it.

The 1542B will certainly get you the most desired features in a PC floppy controller. It supports FM but some controllers do not support 128 byte sectors with the combination of high density and FM. I don't know if I've tried it with that one...but if ImageDisk docs state it supports that combination then I would believe it.

Starting off with 5.25" disks would be a good idea because it will bypass the whole 8" pinout issue. Most of the images you find in the archives will be for double density, though. I don't know of any single density images for CDOS out there, but that might be something I can help with by generating one for you.

The controller can handle both size drives at the same time, but since it is a single density controller none of them can be double density.

Post a clear picture of your 4FDC board and let's make sure it is complete and what switch settings you have.

-- Amardeep
 
Yes the 4FDC was missing a few chips. I robbed the 1771 chip off the TEI floppy controller until a replacement arrives. This board I received has a bunch of mods done to it. I will posts pictures of it tomorrow. I haven't been able to get far enough to actually test the controller since the floppy drives are having problems.
I'm glad to hear that you like the IMSAI/Cromemco combo.
 
Were you able to find a replacement FDC chip to order? They've become quite scarce the last few years. I saw one on eBay but most of the vendors that carried them in the past have sold out. Good thing that TEI had the same part.

There were factory mods for the 4FDC so some should be there. It might also have had a data separator upgrade which some people did to improve soft read error rates. Since your 8" drives are problematic and there is the pinout issue (I'm assuming you don't have Persci drives, they're a bit rare) I'd suggest getting the system up and running on 5.25" first then work out the issues with the 8". Tomorrow I'll look at creating a 5.25" single density CDOS image for you with which you can create physical disks using IMD.
 
a 5-1/4 CDOS IMD disk image would be great. my new floppy controller should be here in 2 days. I assume I will need to write 128byte sectors.
the 8" Drives are Shugart-801
 
Most PC controllers that support FM will do 128 byte sectors for 5.25" data rates no problem. It's only 8" data rates where they sometimes don't. I have one here with that limitation and was quite annoyed the first time I hit it.

I'll try to generate that boot disk now.
 
None of my controllers can write 128 byte according to ImageDisk TESTFDC. It is a matter of getting the S-100 machine (IMSAI) to boot. Then I can copy and create all the disks I need. Here are some picture of the 4FDC controller and the Shugart 801 drives. 4fdc_top.png4fdc_bottom.pngshugart-801_1.png
the bottom side floppy PCB picture is the one that will not seek or do a head load. Although the head load works when the IBM PC does a head seek at bootup.
shugart-801_2.png

I did wonder on these drives if I need to jumper T1 and T2. I know the manual states T3 - T6 need to be jumpered to enable termination on those lines (as the last or only drive on the bus). T1 was jumped on the working drive but not on the non-working drive. Jumping T1 made no difference. T1 is in the middle of the board.
 
Before you get too far into figuring out what is wrong with your 8" drives, take a look at what this article has to say on the topic of using the 4FDC with standard Shugart-interface 8" drives (ie don't bother trying):

Amardeep has a lot of experience with this, and as he (and the article above) have both pointed out, it is recommended you set up your 4FDC controller with a standard 40 track 5.25" drive like the TM100 series used with the original PCs.
 
hmb said:
Click to expand...

Thank you for sharing that writeup. It confirms someone tried the 16FDC RDOS and it doesn't work on 4FDC. That code is probably written specifically for the FD1793.

The 4FDC and 16FDC can both be modified to work with Shugart style 8" drives. In the case of 4FDC it would involve also adding the external data separator mod. For the 16FDC it is just a couple of cuts and jumpers. But that's another bridge to cross once the OP's system is at least up and running on 5.25".
 
Reading from that article:
As a result, Cromemco decided to use the data separator present on Persci 277 drives. This turns out to be less of an issue than expected, as many early 8” floppy drives (the Shugart SA-800, for example) include a FM data separator. But that still leaves the requirement for 5.25” drives that include a data separator…right?
Click to expand...
I will have to do some research if these Shugart-801 drives will work. I will plan on booting with 5.25" right now, but it would be nice to see these 8" drives being used. The other option is to keep my eyes open for a 16FDC for the future
Here is a link from DeRamp. the last page talks about the shugart 800 interface. Sounds like it won't be too bad to get working with the 4FDC.
Here is another quote from retrotechnology.com:
In Jan 2022, Jon Chapman was working on some 4FDCs, and sent this reply to Barry. "... there's a lot of misinformation out there, about the poor old 4FDC. It will in fact do [adequate] data separation on 5.25" drives. You can use any old 5.25" drive with it -- I use Tandon TM-100s with mine. Here's my [2014] writeup. The WD1771's internal data separator isn't great, but it's apparently adequate for 250 kHz FM. Cromemco chose to enable it when 5.25" disk mode is selected, but disable it in favor of 8-inch on-drive separators." - Jon Chapman. (The Shugart 800/801 is an example of 8-inch drives with their own data separators. That fell out of favor in later 8-inch drives. - Herb) Jon has some good suggestions about use of the 4FDC and RDOS and CDOS versions.
Click to expand...
 
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There are some interface pinout changes as well as the need to build and install an external data separator to make those 8" drives work with 4FDC. It's a fair amount of work so you'll have to decide if that's the path you want to pursue.

I'm not having much luck generating the 5.25" single density CDOS boot disk. My plan was to use z80pack to do that but it doesn't seem to support 5.25" disks. My real Cromemco is not running right now so it could be a while before I can do that. I think one of the other Cromemco users with working hardware might be able to help. Let me ask for some assistance on that.
 
From Dave's Old Computers There is the
IMD 300k Cromemco CDOS 2.58 (8" and 5.25")
Inside that are 2 CDOS images. One for 5" and one for 8". I am not sure if these images will work, but those were my original plan once I could write images.
 
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Endersending said:
Reading from that article:

I will have to do some research if these Shugart-801 drives will work. I will plan on booting with 5.25" right now, but it would be nice to see these 8" drives being used. The other option is to keep my eyes open for a 16FDC for the future
Here is a link from DeRamp. the last page talks about the shugart 800 interface. Sounds like it won't be too bad to get working with the 4FDC.
Here is another quote from retrotechnology.com:
Click to expand...
The Shugart 800/801 did provide an on-board single-density data separator. The difference between the 800 and 801 is that the 801 provided a sector separator to support use of hard-sector floppy disks. There is a shorting plug on the 801 so that it functions identically to the 800.
Yes, with some work you could succeed in getting your Shugart 8" drives to work with the 4FDC. However, I recommend you keep on the lookout for a 16FDC (they come up on eBay fairly often) or 64FDC (not easy to find).
Another option to keep in mind is to use a Gotek drive emulator, with Flashfloppy firmware. That will eliminate the need to use a "real" floppy drive, as well as the need to write your own boot floppy on a PC using IMD. Consider posting a request for assistance on the S100Computers Google Group - there are several people there who now use a Gotek with their Cromemco systems. There is also a Cromemco Google Group, but for technical reasons it is hard to gain access for posting to it.
 
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A Gotek might be a good idea since I am getting into non-PC formats. Can you recommend one I would need for my system? Looking on ebay there are tons of them and I have no idea which are real or fake or what will work for what I need.
 
Endersending said:
A Gotek might be a good idea since I am getting into non-PC formats. Can you recommend one I would need for my system? Looking on ebay there are tons of them and I have no idea which are real or fake or what will work for what I need.
Click to expand...

I suggest you buy one that already has the Flashfloppy firmware installed for you. See for example eBay auction number 321429395211 which is being sold by a vendor who has sold a lot of them with an excellent feedback rating.

Everything you need to know about using a Gotek with Flashfloppy:
github.com

GitHub - keirf/flashfloppy: Floppy drive emulator for Gotek hardware

Floppy drive emulator for Gotek hardware. Contribute to keirf/flashfloppy development by creating an account on GitHub.
github.com github.com
 
Endersending said:
From Dave's Old Computers There is the
IMD 300k Cromemco CDOS 2.58 (8" and 5.25")
Inside that are 2 CDOS images. One for 5" and one for 8". I am not sure if these images will work, but those were my original plan once I could write images.
Click to expand...
Almost certainly those are for 16FDC or 64FDC only. Most disk images are in double density format. I've requested single density images from someone with working hardware and one gentleman has responded so let's see if that works.
 
If the 4FDC is single density only, were the original CDOS disks/images for this controller lost?

I ordered a Gotek. I have a Kaypro II that I plan on restoring and a CMC International "Super System II" CP/M computer I bought off ebay that I can't find any information on it, but looks to have tandon-100 floppy drive and has a 8086 processor.
s-l1603.jpg
 
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Cromemco probably stopped distributing CDOS on single density so early in the lifecycle of their systems that chances of a surviving copy anywhere are slim. There certainly isn't anything in the archives older than v2.

I learned long ago that I shouldn't pull the trigger on anything I see on eBay unless I've already established that documentation is readily available. Even so I was burned recently because I purchased a 64KZ-II memory card (which has full documentation including schematics available) but the revision I received was older than the docs and different enough that they were useless. It's just a reality of the hobby.
 
Amardeep-AC9MF said:
Cromemco probably stopped distributing CDOS on single density so early in the lifecycle of their systems that chances of a surviving copy anywhere are slim. There certainly isn't anything in the archives older than v2.

I learned long ago that I shouldn't pull the trigger on anything I see on eBay unless I've already established that documentation is readily available. Even so I was burned recently because I purchased a 64KZ-II memory card (which has full documentation including schematics available) but the revision I received was older than the docs and different enough that they were useless. It's just a reality of the hobby.
Click to expand...
This archive has TGZ format disk images for CDOS versions 00.20 and 1.07 and according to the index these are 5.25" SSSD versions:

This archive has the same versions in IMD format - I have not checked to see what disk size they are for:

deramp.com

deramp.com deramp.com

The definitive Cromemco repository on Github also has these versions, just a bit harder to find among the hugh amount of information available there.
 
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Nice finds! I was going by Github and Dave's Old Computers archives. I didn't realize unix4fun had stuff beyond what was included with z80pack and I had recently browsed through DeRamp so should have realized that one at least. Well, that certainly solves the boot disk problem.
 
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