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Build an internal RS232 Serial AB switch

It did not take me long to discover that I needed some form of serial AB switch to select which external device would have access to the Altair’s main “Serial 1” port. Moving serial cables from one device to another was becoming a bit of a drag. I had to keep switching between my Wyse 55 terminal and the PC I use to handle the “vertical disks” and configuration of the Altair.


I ended up designing an electronic serial AB switch to accomplish this task. The AB switch design would use electronic, not mechanical, switching of the serial signals. The electrical switching design allowed the use of the unused AUX switch on the Altair’s front panel for AB port selection. It is also possible to provide an extra select input that would be used by a remote unit to take over the Altair’s main serial port remotely. This is very handy at times when I am remotely accessing the Altair and forgot to make the switch via the AUX switch.
Serial AB Switch PC Board

In keeping with the look, feel and early methods used by engineers in the 70s and 80s, I opted to design the AB switch using full sized, though hole parts and duel in-line integrated circuits. No surface mount parts or cutting edge processors.


Another design goal is the optimization to fit in to the Altair Clone’s mechanical environment as much a practical. This lead to the resulting PC board being sized and placed to take advantage of the four supplied mounting holes already part of the Clone’s case. These four holes end up close to the extra AUX switch’s connector J6.


I like blinky LEDs. One of the reasons I like the Altair? In keeping with the blinky theme, I have added six LEDs to monitor power, selected serial port, received data, transmitted data, request to send, and clear to send lines. Useful LEDs to help debug troublesome serial problems.
Serial AB Switch Wiring

The integration of the AB switch requires the creation of three new interconnection cables, one new interface cable. Two existing cables are re-routed.


Power to the serial AB switch is supplied via the fan connector J5 on the Clone’s main board. J5 is conveniently connected to the main power switch on the front of the Clone. Two of the same type 3-pin connecters used on the main board are used in the AB switch. Both of these connectors have one-to-one connections passing power between both connectors. A short interconnect cable is used between the main board J5 connector and AB switch J1 connector. The existing fan connecter is plugged in to the other 3-pin connecter J2.


The AB switch has a single larger (0.156) 5-pin serial connector J5. This allows the current “Serial 1” connector on the back of the main board to simply be unplugged and re-routed to the larger J5 connector on the AB switch. This now makes the original DB25 on the back of the clone the A channel. A 5-pin to 5-pin interconnection cable is connected between the main board larger “Serial 1” connector and the smaller AB switch common serial connector J3. The second smaller 5-pin connector is connected to a newly created DB25 serial cable. This new cable is added to the back panel of the Altair case to be used by the B channel serial equipment.


As covered above the unused AUX switch on the front panel allows the user selection between the AB ports. A 3-pin to 3-pin interconnection cable is used to connect the main board J6 connector to the AB switch J6 connector.


The final unused 2-pin connector is provided to allow an external device like a Raspberry Pi to signal the AB switch to take control of the Altair’s main serial.
Serial AB Switch Schematic

The schematic shows the older type electronic used to create the AB switch.  There is even the ever dependable 7805 five volt analog voltage regulator mounted on a big heat sink in place of a more modern switching versions.  To maintain the RS232 serial signal levels three MAX232A ICs are used.  The AB selections is performed by two 74AHC157, Quad 2-input multiplexers.  Selection control logic is handled by a simple RS flip-flop built using 74AHC132, Quad 2-input NAND Schmitt trigger gates. The Schmitt trigger is used to clean incoming signals and a resistor and capacitor is on one input of the RS flip-flop to define the power-up AB channel selected.