Sunday, January 28, 2018

HP 2621A terminal successfully connected to Altair 8800 replica (AltairArduino)


I switched over to my Bluetooth to serial module, since I was having no luck communicating with the USB to serial module from last week. This module, by Parallax, claims that I can talk to it over the serial link, typing $$$ during its first few seconds to switch it to configuration mode, where I will get a CMD prompt.

With the connection set up from my RS232 12V/TTL shifter module to the Bluetooth module, I can type $$$ and see it echoed back but never get the CMD prompt. Incomplete and vague manuals again. This is the trade-off for inexpensive modules where no budget is available for tech writers or usability engineering studies.

I kept bashing away at it, studying the materials from the maker of the board as I attempted to get it to configure. Even when I can talk to it for configuration through the serial end, I have the much more difficult problem of causing the bluetooth in this unit to communicate correctly with the bluetooth module inside the Altair 8800 replica.

At this point, I would be ready to rip out the bluetooth inside the Altair replica and wire RS232 directly from it to my level shifter board. I was not sure if there is any dialog required between the bluetooth module and the Arduino inside the replica, but I my investigation showed that my serial level shifter card could be easily connected in place of the bluetooth module inside the replica.

I tried one more thing before taking the radical step of removing the radio. I removed a jumper on my new bluetooth module, and got it to answer back properly to my $$$ prompt. It was forced to 9600 but I connected, per the instruction guide, at 115200. It echoed my characters but didn't recognize them internally.

Now I had the task of configuring it properly to connect to the other bluetooth module inside the Altair replica. I took a bit of time to do some serious research and plan out the configuration.

I looked up the bluetooth ID of the internal radio in the replica and set up the new external module to attempt to contact it by that address and with the passcode it wants. I was not sure whether it would pair properly, it was possible I needed to be connected to both sides simultaneously.

The new module, in its auto-master mode, immediately connects to the bluetooth module inside the Altair replica. I was able to switch the replica console over to the serial port (i.e. through the pair of bluetooth modules and then into the PC serial port) using the AltairDuino configuration menu. At this point, all I need to accomplish is the wiring of the serial port itself to the Centronics 50 connector for attachment to the HP terminal.

This was built into a small hobby box - bluetooth module, RS232 level shifter and a connection to a leftover wall wart power supply for the 5V to the modules. The cable was completed, including the essential wire between Centronics pins 14 and 45 that brings +5V to tell the terminal that it has a live link.
Building magic box to link Bluetooth to HP 2621A terminal
I brought everything out to the workshop to do a first test, after a final peek at the HP manuals to be assured they were true RS232 signal levels and not TTL limited. I turned on the terminal, powered up the Altair replica and my magic box.

Altair 8800 replica with power supply
Magic box with Centronics 50 connector on left, power supply on right
It worked right away. I used the laptop over the internal USB link to the replica to start configuration, switch the serial port over to the bluetooth module inside, which was live linked through my magic box to the HP 2621A terminal. I saved the config change and tried a few cycles of plugging in and powering up everything.

It all worked well, so I fired up Zork 1 to test out the terminal running this classic game. Worked great and satisfies my objectives. I can now work with a 1970s terminal to speak to the 1970s era Altair 8800 (replica). I swapped virtual floppy drives to restore a saved game and played for a few minutes.

Altair replica running CPM with Zork 1

HP terminal with the CPM session and Zork 1 running

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