Tibbit LEDs and Their Colors
"M" Tibbits have LED indicators. M1 devices have up to four LEDs. M2s can have as many as eight "lights".
LEDs on most Tibbits indicate the state of their control lines. This is why the number of LEDs matches the number of control lines on narrow and wide Tibbits. This is not a rule -- LEDs exist to provide sensible and useful indication of Tibbit states, so they do not "belong" to control lines per se. For example, there are "power good" LEDs on power supply Tibbits (#09, #10, #23, etc.). They indicate the presence of the required voltage on the power line.
We use different LED colors to indicate the direction of the corresponding signal:
Red LEDs are used for output lines (and power "flowing" out).
Green LEDs are used for input lines (and power "flowing" in).
Yellow LEDs are used for lines that work both as inputs and outputs.
Terms output and input should be interpreted with respect to the main CPU of the host board. Therefore, "in" means "from the outside world and into the board". "Out" means "from the board towards the outside world".
As an example, consider Tibbit #14 (DAC, shown above). This Tibbit is based on the MCP4728 IC from Microchip. It communicates with the main CPU via the I2C interface lines SCL and SDA. There are also two additional interface lines LDAC and BUSY. LED colors are:
SCL: this is the clock line of the I2C interface. Red LED is used because this line is an output.
SDA: this is the data line of the I2C interface. Yellow LED is used because the data line is bi-directional.
LOAD: this is the control line manipulated by the CPU. Hence, the LED color is red.
BUSY: this is the status line from the DAC IC. Hence, the LED color is green.
The same logic applies to "power good" LEDs. If the LED is on the power line that provides power to the main CPU/board (power flowing in) then the color of this LED will be green. If this is the power line that takes power from the main CPU/board (power flowing out) then the color of this LED will be red.
Example: Tibbit #12 (+/-15V power supply). This Tibbit takes power from the +5V supply rail and generates voltages for +15V and -15V power rails. Four LED indicators on the module are:
5V power good -- the LED is red because this power is provided by (is flowing out of) the system.
SDWN -- the LED is red because this is a shutdown line that is controlled by the main CPU.
+15V and -15V -- these LEDs are green because the Tibbit provides the corresponding voltages to the board (+15V and -15V power is flowing into the board).
LEDs connected to the control lines are usually wired to light up when the control line state is LOW. On the contrary, "power good" LEDs indicate the presence of voltage on power lines (a "HIGH" state of sorts).
LEDs of Tibbit devices are usually buffered. That is, they do not impose any significant load on the lines they are connected to.
There are no LEDs on "C" Tibbits.