The first challenge is interfacing the Serial AB Switch’s RS232 levels to the RPi 3.3V levels. I achieved this by purchasing a plugin shield board that sits on top of the RPi. As seen here, the board selected is the “SainSmart RS232 / GPIO Shield for Raspberry Pi”. This GPIO shield not only gives me the needed serial interface but also supplied a breakout of many of the RPi’s General Purpose Input Output (GPIO) lines needs in another phase of this project.

The RPi is using a minimal version 2015-2-16 of RASPBIAN. RASPBIAN is just a version of Linux at the core. The initial setup is nothing special so I just followed the process outlined at the Raspberry Pi web site.

Once the RPi RASPBIAN is up and running, I needed to prevent the Linux from using the serial port. The Broadcom UART appears as /dev/ttyAMA0 under Linux. There are several minor things in the way if you want to have dedicated control of the serial port on a Raspberry Pi. The kernel will use the port as controlled by kernel command line contained in /boot/cmdline.txt. The file will look something like this:

dwc_otg.lpm_enable=0 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait

The console keyword outputs messages during boot, and the kgdboc keyword enables kernel debugging. All references to ttyAMA0 need to be removed. The resulting /boot/cmdline.txt, should now contain:

dwc_otg.lpm_enable=0 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait

You must be root to edit this (e.g. use sudo nano /boot/cmdline.txt). Be careful doing this, as a faulty command line can prevent the system from booting.

A login prompt appears on the serial port after boot up and will need to be disabled. This is controlled by the following lines in /etc/inittab:

#Spawn a getty on Raspberry Pi serial line
T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100

You will need to edit this file to comment out the second line like so:

#Spawn a getty on Raspberry Pi serial line
#T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100

Now reboot the RPi for the new settings to take effect. You can use /dev/ttyAMA0 like any normal linux serial port, and you will not get any unwanted traffic confusing any attached devices like the Altair Clone.

I use two different serial software packages to communicate with my Altair. Minicom and ser2sock. Each are accessed using different methodology.

Minicom is a terminal emulation program in Linux. To access minicom I first have to connect to the RPi via SSH and then login to Linux via a user account. From there I run the minicom program within the user shell. This method provides a nice form of data security and limits access rights to only approved users.

Ser2sock is a utility program that allows sharing of a serial devices over a TCP/IP network. It also supports encryption and authentication via OpenSSL. In my setup ser2sock is setup as a simple serial over TCP/IP on port 10000 with no encryption. I just use putty, Tera Term, etc. to access the Altair. For more information on ser2sock visit the project site at https://github.com/nutechsoftware/ser2sock

In my original setup, I used a DB9 to DB25 serial cable to connect the Altair to the RPi. I placed the RPi on the top of the Altair case. Then one day while working on the Serial AB Switch project, I noted all the open space within the Altair Clone’s case. There were still unused mounting holes and the Serial AB Switch was already mounted in side. The AB Switch had just enough 5VDC power needed to power the RPi. It was much neater to route a short remote select line between the RPi and the AB Switch. So that is where the RPi is now installed.

The wire diagram shown details all the interconnections needed between the RPi, the Serial AB Switch and the Altair main board. To understand all that is going on in this diagram and the following picture, you should also look over the Serial AB Switch project descriptions.  There I fill in some of the details of the AB Switch not covered here.