SRB ignition can occur only when a manual lock pin from each SRB safe
and arm device has been removed. The ground crew removes the pin
during prelaunch activities. At T minus five minutes, the SRB safe and
arm device is rotated to the arm position. The solid rocket motor
ignition commands are issued when the three SSMEs are at or above
90-percent rated thrust, no SSME fail and/or SRB ignition PIC low
voltage is indicated and there are no holds from the LPS.
The solid rocket motor ignition commands are sent by the orbiter
computers through the MECs to the safe and arm device NSDs in each
SRB. A PIC single-channel capacitor discharge device controls the
firing of each pyrotechnic device. Three signals must be present
simultaneously for the PIC to generate the pyro firing output. These
signals—arm, fire 1 and fire 2—originate in the orbiter general-purpose computers and are transmitted to the MECs. The MECs reformat
them to 28-volt dc signals for the PICs. The arm signal charges the
PIC capacitor to 40 volts dc (minimum of 20 volts dc).
The fire 2 commands cause the redundant NSDs to fire through a thin
barrier seal down a flame tunnel. This ignites a pyro booster charge,
which is retained in the safe and arm device behind a perforated
plate. The booster charge ignites the propellant in the igniter
initiator; and combustion products of this propellant ignite the solid
rocket motor initiator, which fires down the length of the solid
rocket motor igniting the solid rocket motor propellant.
- PIC - Pyrotechnic Initiator Controller
- NSD - NASA Standard Detonator
- MEC - Master Events Controller
- LPS - Launch Processing System
Here's a schematic of the igniter.
And this shows its location in the forward segment.
The igniter was quite a powerful solid motor itself IIRC. It was expendable of course, but replaceable.
Edit: Spurred on by Tristan's generous proffer of a picture he took of the Safe and Arm device in a JSC lab, I've written up a little more on this device.
Here's his picture (the igniter would be on the right):
Here's a couple of schematics from the 1982 version of the Rockwell shuttle press manual.
From these you can see how the motor rotates the shaft of the mechanism so that the flame tunnels connect the NSDs (aka NSIs) with the charge in the device. And, how the safing pin prevents this from happening when installed.
Technicians removed the safing pins a couple of days before launch - this task required them to enter the forward skirt of the booster. It was part of the "Final Ordnance Installation and Connection" task in the countdown.
Then at T-5 minutes, the mechanism was commanded to rotate, arming the firing chain. At T-0, the cascade of pyrotechnics started: the PICs fired the NSDs, which ignited the charge in the Safe & Arm Device, which ignited the initiator, which ignited the igniter, which lit the booster.
Writeup on the firing chain from online version of Rockwell press manual
RSRM Ballistics Short Course - paywalled
1982 Rockwell Space Shuttle press manual
Example of when Final Ordnance Installation and Connection was performed
Photograph by Tristan