8085 Resurgent: Back to the MAG-85

I took a look at my own website the other day and realized it's been far too long since I have updated certain items there. Most noticeable to me was my MAG-85 project, an 8085-based micro trainer. There are a bunch of things I thought I'd posted over a year ago, but the information isn't there.

Obviously I never did it. Sorry about that.

The front page image for the 8085 project is was a really ugly thing I took at a very interim stage while I was doing regular updates on the project. If I wanted to scare someone off the project, I think that picture is what I'd use. What a rat's nest!

A Very Ugly Looking 8085 Computer

Shortly after that picture was taken I built a real front panel and enclosure. It's been happily living in that enclosure for well over a year, but I never posted the info on it. In fact, I've just started taking it apart in preparation for making some improvements. Since I like to take pictures of my work for documentation purposes (like getting the right connectors back in the right places), I took some photos of the partially-disassembled unit as it is before I make the updates. Here's one:

A bit ugly with the top and bottom panels off, connectors and wires trailing out, but not so bad at the first photo.

Here you can see that it's not so ugly as before. The LED to the left of the LCD display is controlled by the 8085's SOD output. The eight LEDs below the display are controlled by the 8085's OUT 01 command and held in their state by a register. The eight switches below that are on the 8085's IN 01 port.

The program that's running currently reads the position of the switches and outputs a byte to the LED bank to match what it sees on the input. It also reads the keyboard and sends the ASCII + 0x30 character to the screen that it reads from the keyboard (which is in IN 00). The LCD display is on the OUT 00 port.

The four push buttons above the keyboard are, from left (red) to right:
RESET
TRAP
RST7.5
RST5.5

In the crude OS/monitor I have running on the MAG-85 now, TRAP acts as an "Escape" key that returns control to the OS. This allows miscreant programs to be stopped and memory examined any time, since TRAP isn't maskable.

RST7.5 is used as the user vector to the start of the application program in memory. In essence, this is the "GO" or "Execute" button.

RST5.5 is also a user vector that should point to a subroutine that does something and returns. Either the application program should initialize it, or it has to be initialized by hand in the monitor.

The keyboard itself uses RST6.5 to read a key value into a buffer, where an OS routine can pick it up/translate it, etc. The keycap legends allow for several uses of the keys. The typical use of the yellow keys is hexadecimal number entry. But they can also be used as arrow keys and fire button (9) for games.

The top row has the IN or Enter key in red, the backspace (BK) in blue, the (M) mode and edit/view (e/v) keys in gray. The edit/view key toggles a flag in the OS that switches between a memory protecting mode (view) and editing mode in the various modes. The mode key modifies the mode variable in the system to switch between Memory, Register, and I/O port viewing or editing. It's possible to change from editing to viewing and back again while in any of the modes.

Current Rework
My current plans for reworking the MAG-85 have to do with replacing the buttons used for RESET, TRAP and the RSTs, plus improving the ergonomics of the unit a bit by replacing the top and bottom panels, which were hand-made on hardboard, with some nicer CNC'd panels that reposition some of the controls (I switched off the unit more than once when I meant to switch on or off the backlight for the LCD.)

The four pushbuttons above the keyboard are some really awful buttons. They ring like bells, causing all sorts of debounce problems. I have both hardware and software debouncing on them right now, and it *mostly* works. I think part of why I stopped posting before was that I wanted to kill this problem before I posted, so as to avoid causing anyone else the headaches I've had with these switches. And, as you can see, those switches are still in there.

A sideline on the current work is also preparing for a couple of improvements that have been planned since the beginning, but haven't happened yet. One is to put a nice little door in so that the NOVRAM/EEPROM/EPROM memory can be swapped out without opening the whole unit. Right now I unscrew the end pieces then pop out the face panel to get at the memory socket. Which is not clean, quick, or easy. I have to get the cables all to go back to their places each time I put it back together. A couple of times I've pulled out one of the input cables by accident, then wondered why the keyboard or switches aren't talking when I get it back together.

The other is preparing to mount a battery pack inside, so that I can go cordless with this thing. A lot of what I've been doing in design tweaks is reducing the power used by the system. Finding a good brightness for the backlight, putting a switch on the backlight so that I can turn it off when I'm in good light, reducing the brightness of the power LED and the I/O LEDs, that sort of thing. As well as looking at my nascent OS to see if there are things I can do there that will reduce power while staying out of the way of the user's programs.

I'm also planning on adding another memory socket for an EPROM. It won't have a ZIF socket in it like the one that's there now, but I'm getting to the point of wanting a more permanent memory for the OS, with the NOVRAM left for user space programs. This may involve some rejiggering of the memory map, since for mechanical reasons I'd like to have the removable memory in the center of the PCB side to side.

I'm also looking at adding an expansion port or two on the new end plates, which begs the question of whether to use a standard connector with a more or less standard wiring for the port (like a PC's bidirectional parallel port) or whether to roll my own for the sake of less constraint on how the lines are used. Basically just bringing the lines from the I/O buffer registers straight out of the box along with power and ground. This is my preference for a number of reasons, but there's an appeal to letting the MAG-85 drive standard I/O devices, too.

The addition of a serial port, either at TTL levels or a standard RS-232C port is another possibility I keep playing with. I've kept SID available for this possibility, though I was tempted to use it as either a user digital I/O or a memory bankswitch I/O or something of the sort. Being able to connect the MAG-85 straight to a terminal would be really nice, so I'm leaning toward an RS-232C port.

But if I do that, I'll need a separate set of I/O routines for that port that allow it to be the primary user I/O for the OS. So if I do that, it'll probably be deferred until some other things happen first.

Like finishing the OS well enough to post it.

Finishing the MAG-85 Operating System
I haven't posted the OS yet because it's still in a fairly yucky developmental state. That and my test hardware still has the crummy switches that do nasty things to me at times, and there's lots of code in the OS just to try to deal with them that can probably come out once I've got decent switches in.

So my present software objective is to clean up the OS and put a bow on it so that I can "ship" it to a download on my site. I may end up cutting some of the features, but there's also the possibility that I'll end up cleaning them up because I've already got too much other code running around that uses them. The core basics are the ability to view and edit the system's memory, and execute a program starting at some given address. I'll guarantee that much.

I think the ability to view and edit register values is pretty well sewn, too. It's possible to bork the OS by doing something stupid here, but I'm not trying to protect the user from being stupid. Press TRAP (probably to be labelled ESC) and the OS will restart and reset any values it needs to function (I hope!)

The ability to view inputs and edit outputs interactively shouldn't be a problem, either, but it's not an immediate priority. If it happens effortlessly, it'll be in the initial release. Otherwise, I won't hold up the release for it.

I've also got another mode that is enabled in some versions for viewing/setting user variables in the OS, such as the RST5.5 and RST7.5 vectors, selecting different display formats, setting the size of the LCD, and so on. I can pretty well guarantee that the full-up version of this will not be in the initial release, though a cut-rate version of it may be. That would mean that you would set these variables yourself when putting the code on your own MAG-85.

If I do go to a design that uses two memories, one rewritable in system and the other not, I'll have to put these variables in RAM, or expect that they be set once and for all in the firmware. I'd like to be able to provide an EPROM at some point for people who want to build a MAG-85 and get up and running without having to put the OS in themselves. But if I do, I either need to put some system information in the NOVRAM memory space (like the height/width of the LCD display) or require that only one size of LCD be used with a specific version of the OS. Since I'm looking forward to building another MAG-85 with a larger display (perhaps 32 characters wide by 4 lines high), I'd like to keep the OS flexible. The initial display routines will only use a portion of displays larger than 20x2, but the user programs can use the larger displays and the OS can be updated later to have multiple display formats that the user can select.

But right now, I just need to drive a stake through its heart and get a workable version out the door. :)