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MEK6800D1 with 8K RAM, Cassette Interface, and Bit Boffer

R

Random73

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Joined
Jan 19, 2015
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I didn't know other folks were interested in keeping alive their old computers from the 70's and 80's until I discovered this forum just recently. This is my first post to this forum. I'm not sure which forum category this should be in, but here goes anyway.

Last summer I resurrected my homebrew MC6800-based computer, that I built in 1975-77, and restored it to full operation. I also resurrected and repaired my Macintosh 512K to use as the MIKBUG console terminal, running MacTerminal 2.0 and 600 baud. I last operated this computer-and-console combination in 1996. When I first built my computer I also built and used a console terminal for it, using the SWTPC CT-1024 kit, but I sold it sometime in early 80's.

I recently made a demonstration video of my MC6800/MacTerminal combo, available here. In the video you can't see the "Bit Boffer" audio cassette interface card very well, but I am kind of proud of it, so I took a photo of it, included in this post, after I made the video. I am curious if there are other folks out there who also built a "Bit Boffer" from the March 1976 BYTE magazine article, or maybe some other design.

DSC01435cropped60.jpg​

I run the MacTerminal interface at 600 baud, with the character delay set to 2/60ths.

For MIKBUG Object format file transfer from the Mac to the M6800 I set the line delay to 2/60ths.

I use MacWrite to compose source code for the Resident Assembler.

MacTerminal response to the XON/XOFF codes is different from a paper tape reader's response, so to operate the M6800 resident assembler, MP-E, I have to set the line delay as follows
Pass 1P - 60/60ths if I know there are no coding errors
Pass 1P - 180/60ths if I don't know
Pass 2L or 2T - 150/60ths if I know there are no coding errors
Pass 2L or 2T - 180/60ths if I don't know

I also must use OPT NOGENERATE if there are FCC, FCB, FDB directives defining two or more values each.

I hope this post will be of interest to someone.

-73
 
Interests me :)

I am into the SBC stuff also, and I'm not sure where this sort of stuff goes either. I have a 6800D2 kit. It's on a crappy board and I intend to get it mounted on a nice stained board with glass or Perspex cover. Here it is at the moment...

BCisgo.jpg
 
Hi River

Thanks for the reply- Can you use a terminal with your D2? Does it have an external storage interface like paper tape or audio cassette? Did you acquire it in the 70's or more recently?

My tape unit is a Panasonic RQ-212DAS that I bought back then just for this purpose. It still works fine.

After this post I will not have internet access for several days. I will revisit the forum after that, and will describe progress of my current project using my computer/MacTerminal setup.

Thanks again,
-73
 
Hi,

I purchased a lot of mine back in the day and then got more later on. Back in the mid-80s to mid-90s this stuff was regarded as useless and obsolete, but now as the years have passed they are becoming rare. I can use a terminal by re-using the on-board ACIA for comms, instead of tape interface. However, I haven't done this on this particular board as I have other 6800 systems that are more expandable. I am keeping this D2 kit in original format.
 
M6800 ASCII-to-Baudot text converter and teletype printer

M6800 ASCII-to-Baudot text converter and teletype printer

I bought my 6800D1 evaluation kit in mid-1975, soon after they came out. I was in undergrad school at Ga. Tech at the time. I remember the kit was $150, but I have seen other people on internet say it was $300. I would not have bought it at $300. You had to buy the small scale ICs and discrete parts separately, which I ordered from Newark, including the additional MC6810s to fill up the board with RAM. I had access to an ASR-33 teletype at the time, and the campus time-share computer had an M6800 cross-assembler.

In 1978 I obtained and overhauled an AN/FGC-25 teletype set. I made a hardware interface to it from my M6800 computer, and I successfully tested it using a test program. I drew up some flow charts for an ASCII-to-Baudot text converter. Then all was set aside when I went back to graduate school.

This past week I got the hardware interface working again (the 7406 inverter chip had somehow failed during 36 years of storage). Also, I completed and successfully tested a utility program based on my old flow charts. I will be able to upload a text file from MacTerminal (up to about 7 kbytes) to my M6800 and then print it out on the Baudot-code teletype machine.

Now all I need to do is complete the lubrication and checkout of my teletype set, which I started to do last summer. I finished the paper tape reader/reperforator unit, but need to complete the page printer and keyboard.

I had a temporary set-back this week when my Mac became "Sad" with a 05FFFF Sad Mac code at power up, after 7 months of trouble-free operation. I checked the +5V and +12V logic voltages and they were too low. After adjusting the regulator trimmer it is working fine again. I wish my other Mac512K's logic board would have cheered up so easily, but alas, it still always becomes "Sad" with the same error code, 05FFFF, after about 2 minutes of normal operation, even with in-spec logic voltages.
 
My first microprocessor was an MEK6800D2 back in 1976/1977.

It was an expanded unit with a couple of Digital to Analogue converters connected to the parallel ports so I could drive an amplifier or the X/Y plates of an oscilloscope.

The 'fun' that was had hand-assembling programs to enter in hexadecimal and then save on cassette before execution whereupon an error in the mnemonic to hexadecimal code had taken place which corrupted the whole of memory. Re-load from cassette, fix problem, re-save to tape, execute - repeat as many times as necessary...

Exposure to that D2 set the direction of my future career.

Dave
 
daver2 said:
My first microprocessor was an MEK6800D2 back in 1976/1977.

It was an expanded unit with a couple of Digital to Analogue converters connected to the parallel ports so I could drive an amplifier or the X/Y plates of an oscilloscope.

The 'fun' that was had hand-assembling programs to enter in hexadecimal and then save on cassette before execution whereupon an error in the mnemonic to hexadecimal code had taken place which corrupted the whole of memory. Re-load from cassette, fix problem, re-save to tape, execute - repeat as many times as necessary...

Exposure to that D2 set the direction of my future career.

Dave
Click to expand...

Hand coding 6800 , that takes me back to the 70's as well, though I had an ASR 33 , it was at work, so could save to paper tape. The 6800 is pretty easy to hand assemble, nice regular instruction set, can still remember some of it, 86, load immediate A, 3F, software interrupt. My first home brew was a 6809 rig, I saved up for 2 8 inch floppy drives so I could run FLEX, the 6800/09 OS, I had it online ( Janet and bulletin boards ) using an ex GPO 300 baud modem, the size of a small suitcase !. One of these days I must dig it out and get it running again.
DaveH
 
Marty said:
Hi All;
I never Heard of a Bit-Boffer.. What is/was it ??
Click to expand...
Hi Marty,

The Bit Boffer is an interface modem design for data I/O to audio tape casstte. Mr. Don Lancaster wrote an article for the March 1976 issue of BYTE magazine, entitled "Build the Bit Boffer". A .pdf reprint of the article is available on his website. You can see in the article that he targeted the design to be piggy-backed onto an SWTPC serial interface card for their 6800-computer product. The signaling standard he use was called the "Kansas City Standard". There is actually a Wikipedia article about it.

There was another article in that same issue by Jack Hemenway, entitled "The COMPLEAT Tape Cassette Interface". I used both articles for my tape cassette interface.

After I resurrected mine recently, I became curious to find out if anyone else had built a Bit Boffer or used one. This is one reason why I joined this forum a couple of weeks ago, when I discovered it.

Thank you for asking!
-73
 
DaveH said:
One of these days I must dig it out and get it running again.
DaveH
Click to expand...
Hi Dave,
That is exactly what I said to myself for many years. I am just happy I have lived long enough and am still healthy enough that I can resume and enjoy activities that I enjoyed so many years ago. Good luck to you, and let us know how it goes. It took me quite an effort last summer, and some luck, to get my old 6800 computer running again, after all these years in the basement, but it was very rewarding, and fun, to get it back in operation.

Thanks,
-73
 
Hi All;
Random73, I used Cassette interfaces back in the Day, But since I was in the 8080 crowd, I used the Altair 8800 Cassette Interface, and the One Used on Digital Group Machines, designed by Dr. Suding..
I thought that what Dr. Suding Designed was far better than anything else, but I had not really compared it to what else was out there.. I had four choices at the time, My Altair 8800 Interface, The unit designed by Dr. Suding and my AlphaMeca (S-100) intelligent Cassette unit, which I never got to Work, because I had Altair 8800 Memory, which didn't Remember, but the Memory tests said it was OK, And so because of Corrupted memory it never worked, and the Phi-Deck system put out by Digital Group, which I also never got working well, for different reasons..
THANK YOU Marty
 
Oh Bother, there goes tonights project :) ( trying to transfer binary files to my PDP8/m under os8 ), have "dug out " the 6809 sys, will have to give it a cleanup and checkout before trying a power up .
Dave
PIC_0357.jpg
 
The Kansas City tape standard was the de-facto standard as far as I can remember, also called CUTS. Evaluation boards like the Motorola D2 used it. Got some more info somewhere but following a tidy up it has become lost, somewhere. Never heard it called the Bit Boffer though.
 
Hi All;
If I remember right the Kansas City Standard was still like most of the rest stuck at around 300 Baud for Loading Tapes.. But the method for The Digital Group by Dr. Suding You could Load tapes at 1100 to 1200 Baud and I rarely had a mis load..

THANK YOU Marty
 
The KCS interface on my SWTPC 6800 system will control the forward/reverse and the start/stop on two cassette decks. Loading from a cassette at 300 baud was lots faster than loading from paper tape from my ASR-33 teletype.
 
Hi All;
M-Thompson, "" Loading from a cassette at 300 baud was lots faster than loading from paper tape from my ASR-33 teletype. "" Yes, by a factor of about 3X, and so the Digital Group Cassette Interface was about another 3X faster again..
But, I realize that, that is all Past History, and SWTPC 6800 Used what they made both Hardware and Software wise work with their system and wouldn't something from someone else..
But, also Remember that Digital Group had a 6502/6800 Board for their system, But they had Little Software for it, they mostly supported their Z80 Board and to a lesser extent their 8080 Board..

THANK YOU Marty
 
I just got a paper tape reader emulator working in my PDP-8/e. It currently runs at 4,700 CPS, but there is room for another order of magnitude performance improvement. Since everything in the emulator is done in an FPGA and Linux running on an ARM in the FPGA, I could emulate a cassette interface and the computer would not know that it was not real.
 
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