Wednesday, January 24, 2018

Building connection from Altair replica to HP 2621A terminal, rebuilding original Roomba battery pack


The Original Roomba vacuum cleaner that I repaired recently is working well at my daughter's apartment, but the battery pack inside is a bit old and doesn't give a full 2.5 hours of operation on a charge. 

I bought a used battery on eBay, took it apart, and prepared to replace the bad cells with new ones. Roomba uses 12 1.2V NiMH cells inside the yellow plastic battery enclosure, chained end to end. Once a single cell goes bad the entire pack is bad and they have no charge leveling circuitry inside.

12 cells chained together inside Roomba battery case
There are two additional features inside the pack - a fusible link and a thermistor - that provide protection from short circuits and overheating cells. I will install these on the replacement cells.

Rather than buying 12 sub-C sized cells and soldering all the straps to chain them in series, I bought two 7.2V battery packs designed for remote control cars. These have 6 1.2V NiMH calls in them, provide 3800 mAH compared to the original 3500 spec, and two of these will fit inside the yellow Roomba battery case. 
RC battery packs to replace the original cells
Once they come, I will use the fusible link to connect the two packs together, install the thermistor against one of the cells and close up the pack. 


My parts arrived and now I can begin wiring together everything I need to hook my Altair 8080 replica to a true serial based CRT terminal. The chain connects the Native USB port of the Arduino Due inside the replica to a level shifting board that produces 12V RS232 levels, then hooks this onto the HP terminal using a Centronics 50 connector. 

I removed the DB9 connector on the level shifter board and direct wired it to the Centronics connector. The level shifter has to be wired to loopback CTS and RTS on the RS232 level side, since the USB to TTL device produces only RX and TX, it does not handle RTS and CTS.

My cabling will force DTR active, telling the terminal it is connected and cause the terminal to issue CTS to the far side. This may take a bit of fiddling with the control signals on the terminal to get it talking but eventually it will work.

There are commands I can issue to the USB converter board to set its speed, parity and other parameters, which it will store and retain in its onboard EEPROM. I should probably set the converter to the same speed as the terminal will operate, since there are no pacing signals such as CTS to avoid overruns.

Both of my boards need a source of 5V DC for power, which I can either draw off the replica or furnish with a separate supply. The decision is not yet made. I do want to put all this in a convenient box for neatness and to protect the small boards. 

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