Serial Methods for 8088 Experimenter

[Greg Jewett]

So, I did look through existing conversations and did not see an answer to my question.

Looking for ideas to my serial communication queries...

• The MicroUSB on the back of the Experimenter is a serial port.  How would one connect that to a PC with DE-9 serial ports?   
• I then noticed the DE-9 Serial port on the back of the Experimenter?  How do I use this?  Anything special I need to do?

OK, so I will soon have a USB to DE-9 conversion tool (I ordered a new one, since I can not find my previous one... USB to Serial RS-232 Adapter with FTDI Chipset).  

I have an iPad Pro with USB-C.  I have already tried to use a MicroUSB to USB-C cable to connect the Experimenter to my iPad using a serial app.. and it does not see it.  I am hoping I can use the USB-to-DE9 to the Experimenter and that will get seen.

The only PC I have is a tower 486DX running DOS.  Rather not use that, but I could in a pinch.  

What connection methods and solutions am I missing?

Thanks!?

[John Galt]

Part of the reason you can't find anything is because the serial ports can be setup in so many different ways that it depends on you to figure out how you want it.

additionally there are changes to the firmware to allow even more serial ports then the default you see on the configuration menu, but these require changes to the source code to the Arduino firmware on the DUE.

there is also limitations on what software you may use for your O/S.

most people will use CP/M 2.2 or CP/M 3 there are special versions that allow you to change the IOBYTE which is serial port mapping within the O/S but it is limited with off the shelf software.

back in the day most users had 2 serial ports, 1 would feed the terminal and the second was used for a printer or a modem or a null modem connection to another machine or device.

you are also limited with baud rates due to a bunch of factors 9600 baud is your basic go to as it usually works fine.

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The way i use my serial connections.

the micro USB on the back of the Altair-Duino pro works a little different then a standard RS232, I have it mapped to 2sio2 port 1. In order to use the 2sio2 serial ports you need to modify your firmware and enable it.

Without patching the firmware and reuploading to the DUE you can map the microUSB to 2sio port 1 or 2 however 2sio port 1 and SIO is needed for the disk and terminal interface so your left with just 2sio port2 for mapping which would overlap with the other possible serial configurations.

the 9 pin serial RS232 could also be mapped to the same 2sio port 2 or as SIO primary for the terminal. if you have a pro with a GEOFF Terminal that it would already live at SIO primary.

what i like to use my micro USB for is a usb connection to a modern computer i use Terminal software teraterm or putty to transfer files back and forth i modified PCPUT and PCGET to use the higher Serial port of 2sio2 port 1 or 2 over the standard 2sio port 1 or 2. I treat it like a network disk share in this way. just transfer files i need between both computers without disturbing my other connected serial devices.

when using the micro USB on the back you will want to install a USB power blocker. The Arduino DUE will automatically switch to power it sees on the micro USB port and that will bypass your front panel power switch and the wall wart supply and cause problems when you need to cold boot the machine. If you have a large power load on the Aurdino Due with accessories that can overload the DUE USB port using it for main power and could damage it, so i recommend installing a USB power blocker.

Since i have patched the DUE firmware i have access to 4 serial ports instead of the default 2.

I mapped the 9 pin serial rear port to the standard 2sio port 2 this was the default under CP/M for using a modem or a serial printer port. (pin A6/A7) LST: was used for printers and modems (you can think if it like a com port on a PC)

most serial devices connected to this port will require a  null modem adapter to swap the TX and RX lines.

for my setup i have a Serial multiplexer Switch connected to the 9pin rear port that allows me to switch between multiple serial devices on that main port without changing around cables.

I have a serial printer, a serial modem, a raspberry pi, and a connection back to my PC with a RS232 to USB adapter.

the connection back to the pc allows me to use CP/M to dump information like it would be to a remote terminal or a line printer using control-P under Cp/M

its a good way to dump fast information i need to the PC via teraterm or putty and again as a fast external terminal. in a pinch i can also change the IOBYTE and remap my primary terminal to the LST: port and switch back and forth from my primary serial terminal to teraterm on my pc allowing for TEK programs to run. extremely flexible.

the CP/M i use is modified to allow the IOBYTE to be modified(in addition to other modifications). Cp/M has a limited number of main serial ports which can be mapped this is part of the reason i have the micro usb mapped on 2SIO2 port 1 so it lives by itself and software is used to access it on its own.

Additionally i have a TX/RX hack RS232 port that allowed me to install a second 9pin rs232 port on the back of my case. this port is mapped to SERIAL (RXL/TXL) and drives an external FABGL ANSI Serial Terminal that is extremely powerful as comparted to the GEOFF Terminal that comes with PRO kits. The TX/RX serial port must also be enabled with a change to the firmware.

I also have a second micro USB port on the back of my machine that is connected to the DUE on the Native Port (default uses the Programming port for the rear microUSB)

that port is mapped to a hardware Dazzler graphics card emulator and allows Dazzler graphics programs to run on my setup.

to keep things Simple if your using the 9pin serial port on the rear, you will need a null modem adapter then connect it to a serial device. this 9pin port uses 3.3-5v logic and many older serial devices may require 12V power in that case you will need a external serial powered adapter like B&B used to make. i have one of those powered adapters that i use as a serial to parallel converter and it uses external 12V power to feed the connection.

you may need to juggle quite a few different serial port connectors and adapters to get something working but it does work.

there is so much flexibility in the setup you could almost do anything as well.

the easy first step is printer or modem connected to the port. that will give you the ability to output information on paper. a modem will allow you a bunch of options as a modern wifi modem can create a virtual serial bridge between two remote machines. in my case i use the wifi modem to create a virtual serial bridge to a remote Altair computer and i can transfer information between both machines i can even remap a remote terminal and drive one of the altairs remotely as it thinks i'm in front of the machine with a directly connected terminal. wifi modems are a little limited and temperamental so i have a raspberry pi connected to the serial switch as well allowing, telnet, ftp, and WWW browser access to the internet. wifi modems are limited to telnetBBS or other remote machines but there is a bottleneck and they are much slower then your connected baud rates. for fast transfers i just link 2 raspberry PIs together and transfer over FTP. 

i can keep going but you should get an idea of what is possible which is quite a lot.

[Chris Davis]

Be sure you read the posts here: https://adwaterandstir.com/operation/

The DE-9 on the Altair-Duino has a default configuration loaded into config #2 on the SD card you received.  To use config #2, raise SW1, and all other switches down.  That indicates a binary value of 2 on the toggles.  When you raise the DEPOSIT toggle and turn on the Altair-Duino (or connect via USB) it loads config #2.  It is set to 9600 8-N-1.  If you have a serial terminal (or computer with terminal emulator software running) connected to the DE-9 port, it should work.  The serial port can be configured for most baud rates.

Also be sure to read David's complete documentation of his Altair emulator project.  Understand that his documentation may not completely match the Altair-Duino, because the kit is my implementation of his project.  https://github.com/dhansel/Altair8800/raw/master/Documentation.pdf 

[Chris Davis]

BTW, I don't think you're going to have any luck connecting a micro USB to DE9 adapter to the USB on the back of the Altair-Duino.  The USB-to-DE9 adapter expects USB host software to be on the USB side (typically on a PC or Mac), the Arduino does not have USB host software installed.

I've never heard of anyone using an iPad as a terminal emulator for the Altair-Duino.  Maybe I'll give that a try.

On Tuesday, December 2, 2025 at 9:08:13 PM UTC-6 jew...@ejewett.com wrote:

[David Williams]

I experimented with using my Android tablet as a terminal through the Bluetooth module. Since the apps are designed for touch screen input, most of them are line oriented. They have you enter a whole line, then hit send. The few that didn't still had clunky user interfaces for input that didn't entirely go away when using a keyboard.

With the right software, it would work fine, I just never found it.

[Greg Jewett]

@John Galt
Thank you for such a though and detail explanation - but far more than what I wanted to do.  I will admit that the Altair 8800 Experimenter is for me to learn on and learn about.   I never had an original, but my computer science degree did have courses and work utilizing binary and machine language coding - so I can appreciate entering in bytecodes and opcodes via binary and entering a program in that fashion. 

Without patching - I was just trying to figure out the best way to utilize the existing DE9 or MicroUSB serial ports with the default firmware and the kinds of cables people are using to connect this to my MacBook Pro or an iPad Pro (USB-C).  I was curious if this had already been done or if everyone is using a PC w/Windows or Unix.

[Greg Jewett]

Thank you for the information on how to get the DE9 to be acknowledged and working.   Is a NULL modem cable required to connect to the DE9?

I ordered this:  https://www.amazon.com/dp/B00AHYJWWG?ref=ppx_yo2ov_dt_b_fed_asin_title
Originally it was supposed to show up on Saturday - but it looks like it coming in today (yeah!) - so I will experiment with that.   

I am still trying to figure out how I can make the MicroUSB serial port work with my iPad.   I think it would work on my MacBook, just need the machine to acknowledge it as a serial device.  

[Greg Jewett]

My iPad has a full physical keyboard:  https://www.apple.com/shop/product/mwr53ll/a/magic-keyboard-for-ipad-pro-13%E2%80%91inch-m5-us-english-black
So no janky touch combinations or on-screen keyboard to deal with.   I can even map specific keys and function-keys to terminal commands and codes.

I have found several full "console" and "serial" applications that will work with various cables.  Just have not figured out the right combinations yet.  I am kinda hoping the DE9-->USB conversion thing that arrives today will give me serial capability.

[John Galt]

 Is a NULL modem cable required to connect to the DE9?

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depends on device but usually the adapter is required. you can flip the Transmit and receive for the 9 pin serial in the setup for the Geoff terminal but that usually causes different issues and its better to leave that alone and then use the null modem adapter.

i have a 4 port to 1 serial switch box and i have each serial port setup differently. some of my serial ports are setup with null modem, some not, some have different voltages setup with external power supplies as well.

i would recommend just using your macbook and don't bother with an ipad setup.

[Chris Davis]

The VT100 emulator ("Geoff terminal") has no effect on the DE-9 serial connector.

[John Galt]

sorry, i meant the daughter board. i just call it the GEOFF terminal out of habit.