Monday, July 9, 2018

Prepped the Altairduino to drive the ASR 33 teletype for VCF West exhibit


I removed the bluetooth module from the Altairduino motherboard and replaced it with a header strip. This will allow me to plug in the bluetooth module, with its male header strip, or the MAX 3232 board with its male header, depending on whether I want my serial line to operate direct or wirelessly.

I foresee a third option, where I can have a TTL level serial adapter hooked up to the Altairduino - in this case there is no need to convert to true RS 232 voltages. It won't be used with the ASR 33 hookup but might be helpful in other situations.

I built a male header on a small PCB section that also mounted my MAX 3232 module, allowing it to plug into the rear of the Altairduino or to a different female header I will use with the SBC 6120. The small board will deliver +/- 12V receive and transmit signals that have to be connected to my Telebyte 65.

Next step was to permanently wire the small board I built to the Telebyte. The MAX3232 supports two in and two out connections, typically RX, TX, and then CTS/RTS or DTR or other control signals. I don't need any controls so I tied the spare inputs to 3.3V for the TTL in and ground for the RS232 in.

The header where I plug this board provides 5V, but the MAX3232 needs 3.3V supply even though it can handle 5V inputs on the TTL side. The Arduino Due that powers this machine runs on 3.3V thus I know I have access the the proper voltage somewhere, if I can only find it and route it appropriately.

This slightly spoils the symmetry of the socket which I intended to plug the bluetooth or the MAX232 boards as required, since the bluetooth model demands 5V supply. I hoped there might be a workaround, however. The header for these two devices is six pins across, but the active connections on the bluetooth module are only the center four. One end is used for 'state' and the other for 'enable'.

The 'state' pin senses the state of a LED, thus if I can't provide 3.3V through that pin. I looked up the module documentation to determine this. The 'enable' pin actually toggles the module between command and data modes, not something I want to mess with.

This tells me I have no safe pin on the header to deliver 3.3V. Instead, I will need to create a metod on the small RS3232 PCB to drop the 5V input on the pin down to 3.3V for my module. The bluetooth module spec is 50ma at 3.3V. Using a 3.3V zener diode and a 33 ohm resistor I will increase the current draw but the Altairduino is powered by a wallwart supply with plenty of capacity. The diode is on order and should arrive later this week.

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