switches
I want to switch an FTDI interface with a single mechanical switch towards two different endpoints. The FTDI interface has six lines, four two-way digital lines, plus power and ground.
My mechanical switch should be a single SPST or SPDT switch.
With relays, the answer would look like the schematics below. I would need six SPDT relays for that.
I would like to have a smaller solution, with a minimum of components and complexity.
Voltage and currents of my circuit are low, about 3.3V and 10mA.
They all interface to the I/O unit via a standard connector
If the connector has a ribbon cable, identify the problematic signals and cut or re-route these lines on the cable. If this becomes too much of a rat's nest or you don't have access to the individual signals, you could manufacture a breakout board for each processor board to standardize the connections to your I/O module.
I recommend that you use three 4066 analog switches, with the 4 inputs of each chip wired in common with the inputs of the other chips, and the four control lines of each chip tied together and connected to your rotary switch.
Although this requires three chips, they are in a 14-pin DIP package so they are easy to wire up, and each can be selected by a single input (no decode), lending itself to the rotary switch in your diagram.
I have used these same devices (in a SMD package) to switch the four active leads (VCC detect, PGEC, PGED and MCLR) of a PIC programmer, and it works just fine.
The chip has a typical Ron (resistance between the inputs and outputs) of 80 Ω, but that does not seem to affect the operation of the programmer. There are some quad bilateral switches available with a lower Ron, but they are only available in SMT packages.
Best Answer
Use the switch to control analog switches. The switch then just needs to be SPST with a pullup resistor. This will be the control signal to the analog switches. One switch line can easily drive many CMOS inputs, so fanning out to six won't be a problem.
Analog switches are sometimes called "bilateral switch" because, unlike "digital muxes", they work just as well in both directions -- much like a relay. Sometimes I use an analog switch to connect the output of one chip to either one of two "input" lines (with pull-up or pull-down resistors to set the "other" line to the appropriate state), much like a digital demultiplexer. Sometimes I use an analog switch to connect the input of one chip to either one of two "output" lines (no resistors are needed for this case), much like a digital mux. Sometimes I use an analog switch to connect a bidirectional line like the I2C data line.
Analog switches act much like relays, except (a) they can't handle much current -- some handle 25 mA, others can handle only 10 mA. (b) they can handle only a limited voltage range -- some handle 0 to +15 V, others handle only 0 to +5 V. (c) they are lower cost, can switch far more rapidly and more often, and require much less power than relays.
(This is a minor tweak of the answer from Olin Lathrop).
Some parts I would consider:
You might be able to do this with two chips, and perhaps some pull-up or pull-down resistors.