Tibbit Power Lines
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By now you know that the main supply voltage for Tibbits is +5V, and that some Tibbits also need +15V and -15V power. You also know that C1 Tibbits have no standard ground and +5V pins, while C2s do not have a provision for -15V power. Finally, you know that some Tibbits do not require any power. This section continues with the explanation of Tibbit power lines.
The diagram above shows five Tibbits. Two of them are #10 (+5V power supply), one is #01 (four-line RS232 port), another one is #12 (+15/-15V power supply), and the last one is #13 (the ADC converter).
Notice how all five Tibbits have diodes in their power lines. On power consuming Tibbits (#01, #13) these diodes prevent damage to Tibbits if power is accidentally applied in reverse. On power generating Tibbits (#10, #12) the diodes allow you to bank power sources. By having two power supply Tibbits (#10) you can either combine their current output or increase reliability (by having several redundant power sources).
Power, of course, can also be applied to the power rails externally (i.e. from a lab power supply). Our own TPP2 and TPP3 boards have power terminals for direct 5V power input. The boards do not have direct inputs for +15V and -15V. So, although TPP2 and TPP3 have +15V and -15V power rails, the only way to have power on them is to insert the Tibbit #12. You only have to do it if there are any Tibbits in your system that require these optional voltages. On the above diagram, Tibbit #13 does require +/-15V.
One additional point. Power supply Tibbits that provide +5V power generate it from external power sources. For example, Tibbit #10 takes DC input in the 9-18V range, while Tibbit #23 implements PoE (power-over-Ethernet) power supply. Both take external power and convert it into +5V. Tibbit #12, however, generates +15/-15V voltages form the 5V power rail.