Tuesday, January 16, 2018

Using HP 2621A terminal with Altair 8800 replica


I recently bought and built an Altair 8800 replica - uses an Arduino and front panel interface - which I access over bluetooth to a modern PC running a terminal emulator. The modern hardware distracts from the sense of using the Altair, so I decided to connect it to one of my ET style HP terminals.
Arduino based Altair replica
The 2621A is called an ET type because the shape of the monitor head on its pedestal looks a bit like the alien from the ET movie. It runs over RS232, usually hooked to my HP 1000 minicomputer system. However, it could be driven by any system, using ASCII input and output.

HP 2621A "ET" terminal
A few minor difficulties sat between me and immediately connecting the terminal:

  • Lack of a serial port on the Altair
  • 12V levels on a real RS232 interface
  • Terminal uses a Centronics 50 connector
  • No cable to make the connection. 

The Altair replica does not have a real RS232 port, although it emulates the serial ports of the 88SIO, 882SIO and 88ACR cards from a real Altair. However, it has two micro USB ports on the Arduino and a bluetooth module inside. There is a simple configuration program built into the replica to assign these real ports to the virtual Altair serial ports.

On a recommendation from other builders of the Altair replica, I ordered a device from Hobbytronics that will plug into the Arduino native USB port and produce TTL voltage level RS232 signals. This gives me the protocol but not the voltages.
Convert native USB in CDC mode to TTL RS232
I then procured a MAX2232 chip based level converter which interfaces a TTL level RS232 device such as the Hobbytronics box with a 12V level RS232 device. This gives me the voltages but not the cable or connector.
Convert TTL RS232 to true 12V RS232
Next, I have a male Centronics 50 connector on order, which I will wire up between the level shifter board and the terminal. That resolves the last issue and provides everything I need to begin using the 2621A terminal with the Altair 8800 replica.

Centronics 50 male connector plugged into HP 2621A terminal
The bluetooth connection in the replica will let me transfer files to act as cassette tapes, paper tape or xmodem mechanisms to get external files into the Altair replica. It has simulated floppy and Pertec hard drives inside the replica, hosted on an SD card, thus I can do much of what I want without the need to access virtual paper tape or other files from outside the replica. 


My home office needed a makeover now that I am retired and we want to use the space for storage. One element of the refresh is to replace the carpeting that is old, ripped and stained. The room is 14' 10" x 8' 3", a simple rectangle shape.

My wife and I selected a carpet that comes in both 12' and 15' wide rolls, since that way the room could be covered with no seams at all, just a 8+ foot length of 15' carpet. We bought from Home Depot and had the tech out to officially measure the room. Next step was to get a quote. Based on the carpet we chose, priced by the square foot, we guessed that the total cost would be in the 400-500 range.

In spite of the use of square feet in all the advertising and quoting, carpet is actually sold by the yard and you pay for the entire width of the roll times the length needed to cover the room. Knowing that, I expected that I would pay for 15 square yards - a 15' wide roll is 5 yards wide and the nearest size in yards to cover the 8+ foot room is 3 years- thus 5x3 yards of material.

We received a call with our quote which was for over $600. This was for 20 square yards of carpet. That told me that they had simply chosen the 12' wide roll, cutting it for the 15' length of the room. 20 square yards when all it would take is 15 square yards if the wider roll was used.

Fortunately we knew what was happening and could ask them to recalculate using the 15' roll. As expected, the cost came out right where we expected give that it now required 9 feet to be cut from the 15' roll. The deal was concluded and we expect the 15' segment, a 'special order' not in stock, to arrive at the installers in about two weeks after which they can come and install it.

We saved 1/3 of the cost of this job because we knew how they worked and the best way to lay the carpet. Even though when we first ordered the carpet we specified the 15' wide roll, and when the tech came for measurements we asked him to note that 15' rolls should be used, the quote desk approached this in a simple minded way that ballooned the price unnecessarily. 

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