Most of the older aircraft use very old equipment, and that in normally Analogues. These pieces of equipment need to be repaired or replaced with newly built equivalent equipment. This covers Aircraft and Rail.

Digital came after the Analogue Electronics and some older types of aircraft use a hybrid of Analogue and Digital electronics.

There is still some equipment that uses Thermionic Electronic Valves in their circuits. Valves were becoming scarce later on, after the transistor era started, but are now making a comeback.

The Transistor made a large impact in the 1960’s, replacing vale circuits at lower voltages. The higher current Valve circuits were replaced with a later transistor known as FET’s. (Field Effect Transistors.) These would work with lower voltage and higher currents. Valves would use a typical voltage on average of 350Vdc, whereas the transistors would work at an average voltage of 12Vdc low current to 60V High current. The transistor was invented in the 1940’s by the military to make their communication radios easier to carry.

All these changes need to be taken into consideration when repairing or building new equipment.

Later on, in the early 1070’s, the Integrated circuit arrived. This now changed the size of equipment once again. Instead of using 20 or more transistors, an integrated circuit would do the same job. Smaller and easier. As of 2020, the most transistors (MOSFETS) in an integrated circuit are 12.6 American Trillion. (That is 12,600,000,000,000).

Sometimes, when aged equipment is sent in for testing or requires a new replacement, new test equipment needs to be designed and built to perform all the functions of the said equipment, as even now, the old test equipment and documents do not exist anymore.