Tag: S-100

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More Users and TUARTs

Posted on by Graham

Cromix Plus is a multiuser system, but to support more users requires more serial lines and terminals.

I had a Twin UART (TUART) card that would fit the bill and i’d purchased one more for another project, but it can be plugged into this rack for now. This would give a total of 5 users.

As for terminals – well they are hard to find so i have to be satisfied with a Windows machine with several serial ports and multiple instances of kermit.

The TUART is installed as TUART #1 as per the Cromix Plus Administrator’s Guide.  This puts the ports at 20h and 50h and makes them 2 and 5 in the sysdefs file.

Cromix has been regenerated to enable the two TUART serial ports as devices tty2 and tty3.  The disk controller serial port remains at tty1.

The TUARTs support interrupts, so the 16FDC no longer has the interrupt line to itself. Priority is established by a daisy chain that connects from the OUT on the 16FDC to the IN on the TUART. Just to mess with everyone’s heads, the IN and OUT positions on the TUART are the reverse of the 16FDC.

The serial ports can be configured as terminal ports by editing /etc/ttys:

Along the way, I stumbled on a Software Update Note for 20.14 that said:

This means that if no terminal is connected, the port should not be configured as a terminal port.

A second TUART was setup as TUART#2, but this clashed with the IDE card, so it was moved to TUART #3.  These are 8 and 9 in the sysdefs file used to rebuild cromix.

The second TUART also needed to join the interrupt priority chain: OUT from the first TUART to IN on the second TUART.

Here’s an example of multiple users (3) and multiple processes (9):

(This pic is from after i increased the memory.)

More Memory

Posted on by Graham

Wishing to be able to run the disk checks properly to be able to run lots of processes, and haunted by the thought that more memory failures can be expected on the 512MSU, i again turned to a modern solution.

This time it was a static memory board designed by John Monahan:

http://s100computers.com/My System Pages/16MG RAM Board/16MG RAM Board.htm

I could have gone for the full 16MB and filled the address space but i wanted to keep the 512MSU so i built an 8MB version. The existing memory could be moved just by changing some DIP switches on the MCU.

This project would probably still be pending but for a bare board being up for sale in South Australia.

If a generic memory board is used instead of the MCU/512MSU then:

  1. It must support a Phantom signal
  2. The 16FDC needs to be modified to provide a Phantom signal
  3. The Phantom signal needs to either be on a different pin from the normal 67 or the DPU needs to be modified to not use pin 67 eg by cutting pin 6 of IC 41.  It’s probably better to modify the new board to use pin 69 and be consistent with other group members.

https://groups.google.com/g/cromemco/c/T_eui2YJEQ0/m/FkrSFaYtBwAJ

I think it may still be possible to use the DPU as well if the memory card is 4MB or 8MB.  It just needs to be located after the new board.  It can’t go before because the board boundaries are not fine enough.

The SRAM is expensive at over $10 per MB.  In the end it was easier to get the 4MB parts and two are required so I built an 8MB variant.

This card will be located at 0000H-7FFFFH. The 512MSU will be at 8000H-87FFH and the DIP switch will need to 01000.

The biggest challenge with building the board is mounting the TSSOP packages.  I used paste and hot air and cleaned up the excess with solder wick.  There were some troublesome bridges one of which only cleared after reflowing with hot air.

I had a look at the PAL equations just to make sure they would work with 8MB.

GAL5:

/BOARD_SELECT  =  sINTA  + sINP  + sOUT  + /PHANTOM@

bsMEMR@ =       /bsMEMR

GAL6:

/S100_8_RD_OE@      =      /RD8@  * bA0   * BOARD_SEL * /pSYNC                  ;U19, 8 Bits to CPU, odd/high address
                         + /RD16@         * BOARD_SEL * /pSYNC                  ;Any 16 bit data Read
                         + /WR16@         * BOARD_SEL * /pSYNC                  ;Any 16 bit data Write

/S100_8X_RD_OE@      =     /RD8@  * /bA0  * BOARD_SEL * /pSYNC                  ;U31, 8 Bits to CPU even/low address

There seemed to be no interest in the address lines ie this board is selected regardless of the address (except for the phantom).  For a 16MB card that would be correct. And it doesn’t matter for writes as long as all of the address lines are in use at P101 – this should just cause a write to RAM that isn’t loaded. But it would cause a read contention with a board in the top 8MB eg where I want the old 512kB to be.

I think that for my situation GAL5 so that /BOARD_SELECT is asserted only when the for the first 8MB.

/BOARD_SELECT  =  sINTA  + sINP  + sOUT  + /PHANTOM@ + A23

I made the change.

It took hours to run the 68k RAM test in CDOS, but it was successful. 8.5MB of RAM was good to go.

cromix.sys had to be regenerated with an updated sysdef. After this there was abundant memory! There are a bunch of tweaks that i could do to take further advantage of the memory and i think a RAM disk could also be added.

8085/86 Getting Started

Posted on by Graham

This machine is a reconstruction of a hobbyist built machine from the early eighties. It came to me in pieces in amongst a lot of other gear, so it wasn’t obvious to me that there was a complete computer there at all.

I had already rediscovered a Cromemco based computer which used half a dozen of the 29 S-100 cards that i’d received. I also had a rack, some 8″ drives, and a lot of 8″ disks which i had previously imaged.

Many of the disks were labelled as being Jade, and there was indeed a JADE DD Floppy Disk Controller amongst the cards. There were many more disks that used the JADE format.

There was also a Versafloppy II Floppy Disk Controller card, for which there were also a number of disks.

These were all CP/M disks so i was looking for a 8080, 8085 or Z80 processor board. The candidates were a Cromemco SBC which wasn’t a great fit with CP/M or a CompuPro 8085/88 CPU card which would work.

I also expected there to be some I/O and a card for a boot ROM. And RAM of course. There weren’t a lot of clues from the cards themselves.

Fortunately there was a disk labelled “Jade System BIOS Development which contained some assembler files for the CP/M bios and the boot ROM. At this point, i ruled the Versafloppy II out.

The boot ROM code had some comments identifying key components (8251, 8255 and 8253) on a W/W card. Once i found the card with that combination of components, i realised that W/W stood for wire-wrap. It was a hand-crafted board. It also has a speech synthesiser chip!

Looking at the chassis, i could see that the wires dangling from the back panel married up with empty sockets on the board.

A little while later i realised that there a couple more ports which may belong to this unidentified card:

It has a real-time clock on it.

I had several memory cards that could potentially be used, but given that the CPU card is capable of addressing more than 64k, i started with an Intersystems 256KDR.

This card is probably overkill, and i may swap it out in the future.

I do have an EPROM card, but it is made for 2708 EPROMs which i can’t program at present – i would need a new programmer.

Instead, i built a new card using a modern S-100 prototype card.

These cards for the basis for the system.

Jade Double D FDC

Posted on by Graham

The Jade Double D is notable for being an intelligent floppy disk controller. The onboard Western Digital FD1791 FDC chip is controlled by a Z80 processor. The CPU communicates with the DD through a 1kB window.

Although the card includes a processor and 2k of RAM it has no ROM. Instead, code must be injected by the CPU card. The code is embedded in the CPU boot ROM.

The boot code assumes that the window address must be set to F400. The jumpers were already correctly set.

This card supports 5.25″ drives on a 34 pin interface or 8″ drives on the 50 pin interface. Eventually i will use 8″ drives with this machine, but in the short term i just wanted to use a couple of goteks (with Flashfloppy).

The images that i wanted to use are from 8″ disks. These work fine in the gotek but because the gotek uses the 34 pin interface there were a couple of residual issues. There are a some signals expected by the boot ROM and the CP/M BIOS for the 8″ drives that are not on the 34 pin interface.

The first is the RDY signal which is supported by Flashfloppy on pin 34. This requires a link on the DD from pin 34 of the 34 pin interface to pin 22 of the 50 pin interface.

The second is that an 8″ drive can detect whether a disk is single or double-sided – the index hole is in a different location. For this, i added a switch to assert or negate the SIDES signal depending on which image i had loaded. I later copied all of the single sided disk images to double-sided disk images to make things a little simpler.

This card had a couple of hardware issues. The first was two shorted tantalums. The second was that, for whatever reason, two sockets had been butchered, and the chips soldered to the socket pins.

I replaced the sockets and devices.

An EPROM Board

Posted on by Graham

I needed an EPROM board to complete this computer and i could not see any easy way to obtain an eighties era card, so i had to knock one together myself.

I started with an S-100 Prototype board designed by John Monahan. This has all the buffers etc that end up on nearly every S-100 card, so a lot of the hard work is done.

http://www.s100computers.com/My%20System%20Pages/Prototype%20Board/Prototype%20Board.htm

John’s board designs are available for anyone to produce (as far as i can tell) and they are easy to get in the States. Not so easy in Australia, but on this occasion a batch turned up in Australia so a bought a few.

I had what i thought was the boot ROM code on an 8″ floppy disk. I’d previously extracted the files so i could have a good look.

The boot code indicated that the boot ROM would be located at F800 and occupy 2k. The Jade DD floppy disk controller uses a 1k window at F000. CP/M is set up for 60k, so there is also 1k free at F000. The boot allocates this to a Micropolis boot ROM similar to the one in the Sorcerer/Micropolis System.

I allowed for an 8kB EPROM (2764). J1-3 sets which address lines are used for decoding.  To get just the top 2k, J1=J2.

Switch 1 sets the address of the ROM: in this case 00F800: 0000 0000 0000 1111 1000 0000 0000.

The highest address line decoded is A18, so the ROM will reappear at 04F800 but I don’t expect to ever have that much memory. That allows room for 256k of RAM.

My design worked immediately … no, of course it didn’t! I made several mistakes and i had trouble driving the phantom line (which disable RAM at the EPROM address). I eventually settled on the following:

I programmed an EPROM with the boot code that i found on the floppy disk.

There’s plenty of space left on the prototype card for some I/O if i ever run out of things to do.

8085/88 First Boot

Posted on by Graham

The cards were popped into the rack, including a partially tested EPROM card with a boot ROM programmed using a file from one of the 8″ floppy disks.

The source showed that on start-up the first serial port to receive a space character would become the console and offer to boot from the Jade DD.

The boot ROM source indicated a serial port speed of 9600. I used an IBM terminal setup accordingly.

I started with a CPU, EPROM, and I/O cards and proceeded to discover the various mistakes i had made with the EPROM card!

Once i could see that code was being read from the EPROM i added the RAM board and the Jade Double FDC. I could see data coming out of the UART, but it wasn’t making it to the terminal and that was because a shorted tantalum on the Jade board had blown the -12V fuse.

Once repaired, i got the boot screen.

I hooked up a couple of goteks and on the second image it booted.

Credit to the original builder/owner/operator.

Using HD 5.25″ FDDs as 8″ Drive Substitutes

Posted on by Graham

High density 5.25″ disks have much the same capacity as 8″ double density disks, and they operate at the same data rate. A 5.25″ FDD typically has a few more tracks than an 8″ FDD.

This means that a floppy disk controller expecting to see an 8″ disk drive can potentially be fooled into working with an HD 5.25″ drive. This can be very handy if you don’t have an 8″ drive or if you want a more compact setup. This trick seems to work ok with a real HD drive or with a gotek/flashfloppy. 8″ disk images can be written to HD 5.25″ media without alteration using Greaseweazle, for example.

There are a few small issues. The first is the 8″ drive interface is usually 50 pins and the 5.25″ interface is usually 34 pins. The disk controller may have both interfaces (eg the Jade DD and the 16FDC) but not always (eg the Pulsar Little Big Board only has a 50 pin interface). If there is no 34 pin interface, then an adapter will be required. The 50 pin interfaces vary a bit, so a specific adapter may be required.

The second is that 8″ drives can detect whether a drive is single or double-sided and tell the host. The operating system driver may exploit this information (eg Jade DD CP/M) so there may need to be a way to fake this. For systems that use only single sided or only double-sided disks the signal can be tied appropriately otherwise a switch may be required – and if there are different drives in the system then it may be necessary to take the signal low through a diode from the drive select line.

Third is that 8″ drives typically produced a ready (RDY) signal. Without this signal, a host may just hang. This signal is available on many drives and can usually be setup on a gotek/flashfloppy but it may not be connected on the 34 pin interface. This can be overcome by connecting pin 34 on the 34 pin interface to the appropriate pin on the 50 pin interface.