SWITCH SELECTOR

All stages, and the IU, are equipped with a switch selector. This unit has electronic and electrome­chanical components which decode LVDC/LVDA sequence commands and switches them to the prop­er circuits en each stage. This system has several advantages: reduction of stage interface lines, in­creased flexibility with respect to timing and se­quencing, and conserving the discrete output circui­try in the LVDC/LVDA. Sequencing commands can come as fast as every 100 milliseconds.

Stage power isolation is maintained in the switch selector by using relays as the input circuit. The relays are driven by IU power, while the decoding circuitry and driver output are powered from the parent stage. Input and output are coupled through relay contacts. These contacts drive a diode matrix used in decoding the 8-bit input code to select the output driver, producing the switch selector output.

There also is a check and proceed system built into the switch selector. After the switch selector re­lays have been “picked,” the complement of the received message is fed back to the LVDA/LVDC where it is checked. If the feedback is good, a read command is issued. If there is disagreement, a new message is sent which accomplishes the same func­tion. (Note: For redundancy, two messages’ codes are assigned for each switch selector output).

Electrical System

The electrical system powers the IU’s equipment. As with most of the IU’s systems, the electrical system is divided into two sectors: prelaunch and flight. Ground sources provide power through the umbilical lines before launch. At approximately 25 seconds prior to liftoff, power is transferred to the four 28 VDC IU batteries. Each battery has a 350 ampere hour capacity, and loads are equally distributed to drain.

Two special power supplies are provided: a 5-volt master measuring voltage supply converts 28 VDC main supply to a highly regulated 5 VDC for refer­ence and supply voltage to the measuring com­ponents, and a 56-volt power supply for operation of the guidance system’s ST-124-M inertial platform and the platform’s AC power supply.

In order to get the most out of the battery stored power during flight, the LVDC and LVDA turn off unused or unimportant circuits in favor of more important applications as the mission progresses.