The SSME and SRB ignition signals are a result of software flags which are generated in the redundant set launch sequencer (RSLS). For the SSME's, the flag is interpreted by the SSME software operation program (SOP) and sent to the engine interface unit (EIU). For the SRB's, the flag is interpreted by the master events controller (MEC) SOP and sent to the SRB ignition pyrotechnics. All of the times are referenced from an expected time of lift-off (GMTLO, V99W8801C) that was sent from the launch processing system to the RSLS. The RSLS subtracts the GMTLO value from its current value for GMT; this difference is the high-frequency executive read of GMT completed 54 milliseconds prior to the start of the 40-millisecond cycle in which the RSLS is running and checks to see if the difference is less than 6.600 seconds. On the first pass that the criteria is satisfied, the software flag for main engine 3 start command is set. The calculation of this difference is not made again, and the last value is held in the telemetry stream (V90W8380C). The timing is such that 6.600 seconds after this time, the software flag is set by the RSLS for the MEC SOP to fire the SRB ignition pyrotechnics.
Forty milliseconds after setting the software flag for SRB ignition, the RSLS logic sets a flag for T-0 umbilical-release fire commands. The MEC SOP translates this into a command to the MEC for T-0 umbilical release. The MEC splits this input into T-0 umbilical release and holddown-bolt pyrotechnics fire commands which leave the MEC 40 milliseconds after the SRB ignition pyrotechnic fire commands had left. Following the 40-millisecond cycle, the RSLS sets a flag to reset the MEC and then terminates operation.
