combining digital circuits

1.41.1 Many electronics hobbyists combine all sorts of digital circuits into works to be marvelled at. However, even they sometimes have that uncertain feel- ing: must they all be powered by one unit or should there be more or can there be more? And in what sequence should they be switched on? Printer first, or computer first? In digital engineering, which by defi- nition embraces computers, inputs are driven by outputs: information is being transferred. When the IC that drives has a power supply, but the receiving one has not, a current will ensue, whether the circuits are TTL or CMOS. This is an undesirable situ- ation, although it does not normally lead to damage. But the ensuing cur- rent may be so large that the IC pro- viding the current does not operate efficiently any more, because its out- put voltage, owing to the large cur- rent, becomes too low. Particularly bistables can become disorganized by this. It is, therefore, possible that a certain equipment does not work pro...