This analysis isn't complete. There were different series to fire external flash and i'm not sure that i've logged all of them. I've tested with Dynax 60 film camera and Minolta 5400 HSS external flash unit.
Basic idea of the film protocol that external flash fires in high speed mode (pulses with 1/32 (?) of full flash duration) until TTL-OTF sensor in the camera stops by internal flash pulse. One test did show that after ~ 2/3 of full flash (20/32) pulses - external flash fires all remaining power in one big flash (i think that is because there may be no enough voltage on the flash's capacitor to fire after about 3/4 pulses with 1/32 duration so it just fires in one pulse to its end).
Frequency of these pulses is 2500 Hz (0.4ms delay between).
"Main flash" series (only internal flash), 23 ms delay from beginning of the series:
"HSS flash" series (not sure that is really HSS flash start series, could be "TTL-preflash" too; only internal flash):
"Main flash" series (recorded with external flash unit):
"Main flash" series with "Stop" pulse (internal flash and small amplitude impulses from external):
Pulses from external flash during after "main flash" series (only external flash):
Hope, someone will able to complete this.
Vasim V.