Two serial ports?

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Two serial ports?

Postby Frotz661 » Nov Tue 03, 2015 2:00 am

Has anyone here successfully modified their machine to support two serial ports? I'd like to investigate making a tape interface. It seems that the original Altair tape interface was merely a serial board with a simple modem bolted to its back. The one serial port I have comes off the ramdisk board. If I add the serial card kit to the mix, will I then have two usable serial ports? It it possible for the Micro Altair to use a yet-to-be-designed serial board that provides even more ports? I gotta have one for a printer and one for a modem too, heheheh.
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Re: Two serial ports?

Postby jasonrp7743 » Nov Tue 03, 2015 7:37 pm

This is what I am trying to find out too!

Is it the case that, the serial port that is on the RAMDISK (and also the external serial port that was included I think on some kits) is only for connecting an external terminal if you are not using the onboard terminal (onboard VGA and PS/2 interface)?

Or, can it be used to connect a printer or modem while using the onboard for video and keyboard?
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Re: Two serial ports?

Postby Frotz661 » Nov Tue 03, 2015 8:10 pm

As I was doing the final assembly on my machine, I was tracing lines to rule out shorts. At that time I noticed that the traces for the serial port on the ramdisk board went directly to pins on the expansion connector. Then I looked at the schematic. That serial port is actually on the main board and feeds the PocketTerm. So, that serial port cannot be used for anything but a console. Casually remembering the schematic, all the data and address lines are present. There should be enough there to make as many real serial ports as we want. I don't know yet what other bus signals are required. The next step, I suppose, would be to study some S100 serial board schematics and see if what was done there could be consolidated into fewer chips. MAX232 chips should be a big help.
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Re: Two serial ports?

Postby jasonrp7743 » Nov Wed 04, 2015 8:24 pm

Check out the book "Build your own Z80 computer", page 138 (Basic peripherals chapter, section III serial interface)

A quick google search shows this book available in google books, and I downloaded a copy in .pdf form that has in the front page the following quote:

"Please note: The original work was only available as a hard copy. Thanks to Andrew Lynch and Bill
Bradford for their work in creating the PDF and getting permission from copyright holder Steve Ciarcia
to release it. Scanning done by Bill Bradford."
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Re: Two serial ports?

Postby jasonrp7743 » Nov Sat 07, 2015 10:18 pm

I'm really excited about getting a serial card together for these computers. I can't stress enough how much this would add to these cool little computers!!!

I'm still reading up and learning about serial ports.. I found this youtube: https://www.youtube.com/watch?v=JJZOTtwpAjA

So far, I'm thinking a simple board, that interfaces with the expansion connector, and that contains a 16550 UART with an external crystal for a baud generator, a couple other chips for RX and TX buffers, and a Max232 connected to a DB25 connector might work..

On the other hand, I can't help but wonder, would it be easier to build a board with an ATmega328, with a little bit of software to buffer up 8bit chunks of serial data, put in the start/stop bits, use the DATA lines 0-7, and handle to proper signals to R/W and IOSEL?

What do you guys think? This might be way simpler/cheaper/easier than building up a homebrew unproven design, or a replica of the actual MITS serial card, unless the ATmega idea I have won't work due to timing or other considerations..
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UPDATED: Two serial ports?

Postby jasonrp7743 » Nov Thu 19, 2015 9:27 pm

Here's where I am so far...

I've come up with a theory, that, by using an Atmel AVR, I could design a very simple interface board (simple meaning, easy to design, cheap, low parts count), yet functional serial interface. There is just enough hardware here (I think!) to hard-code the I/O ports to the computer. If the computer attempts to use address 246 or 247, and the IOSEL line is high, it's talking to this board and not it's memory.

The Data Bus, IOSEL and RW signals are either passed to the AVR directly, or passed through an inverter if the "on" is low, so that all signals are low for 0, and high for 1. The AVR would then run custom code to make it act like a UART. Finally, a Max282/DB9 connector are also connected to the AVR.

This is all very theoretical of course! I'm hoping people here might want to collaborate on this project. I've attached the schematic, and a current rendering from the board design.

Thoughts? Think it will work? Any good coders/hardware hackers out there want to work together on this?

Image
Image

Jason
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Re: Two serial ports?

Postby Frotz661 » Dec Sat 12, 2015 5:38 am

For your board, I'd eliminate the second expansion connector and instead use a long-tailed socket. It would make stacking up more boards easier. The usual footprint of a DE9 connector pigtail is a 2x5 header, not a 1x9. If the memory and ramdisk board is made mandatory, then the serial board could be made the full width of the motherboard. That should give plenty of room to do at least two ports with traditional UARTs.
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Re: Two serial ports?

Postby Frotz661 » Dec Sun 13, 2015 4:04 am

Here's something using a 16550A UART: https://github.com/DavidGriffith/altairmicro-serial

I'm not at all sure it would work and would very much like comments and advice.

Vince? Are you still around?
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Re: Two serial ports?

Postby jasonrp7743 » Dec Sat 19, 2015 6:28 pm

If someone beats me to it, I'm all for it, as I just want an extra serial port for this computer! :-)

I've got the components in now, and will be breadboarding shortly, which I think at the very least getting the board to realize when the computer is trying to talk to it as a serial device should be easy (will prove it out though, with a BASIC program and LEDs...)

Then, I will start coding the Atmel chip- same as used in an Arduino.. I will program it to behave as a 16550A UART.. The goal is, low power requirements, and low parts count. I want to be able to use this new serial device in addition to the RAMDISK board, and the original extra serial board (which appears to only provide a different terminal connection, but cannot be used in parallel with the onboard terminal).. I figure with those two original add-ons, we have to be getting close to the 800-1000ma that the original power adapters provide.

Anyone know if a dot matrix printer would connect via serial? Or were those parallel. I can't remember.
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Re: Two serial ports?

Postby Tor6502 » Dec Sun 20, 2015 3:37 pm

Dot-matrix printers could be serial or parallel or both. My Oki printer (another Epson mx80 compatible printer) has a parallel interface but also a slot for an optional serial board. Which I also have, fortunately.
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