The TiVo Premiere appears to use the same TTL serial interface as the Gen06 platform (TiVo HD). The unit uses 5V TTL logic to communicate via serial, and any adapters that work with the TiVo HD will also work with the Premiere.
Without any modification to the bootpage, here is what factory TiVo Premiere displays upon power-on:
Access to the PROM Boot Menu is no longer restricted by a factory password unique to the unit. Sending ^C to down the line during the first few seconds will display the PROM boot menu.
G=00000000 R=0000000D W=00000008
TiVo Gen07 release 1.00 (2009-10-20 14:09:37)
Copyright 2009 TiVo Inc. All Rights Reserved.
TSN: REDACTED BREV: 0x1060 MAC: RE:DA:CT:ED
Disk: WDC WD3200AVVS-63L2B0 F/W: 01.03A01 S/N: WD-REDACTED
Booting from partition 3... 3800064 bytes
Kernel signed by '... the Porridge bird ...'
Hashing kernel (SHA256)... done
Checking signature... done.
Signed, valid for release
Kernel entry point is 0x802b8000
cfeBootParms ===> root=/dev/sda4
Kernel boot options: root=/dev/sda4 console=ttyS0,115200 boardID=0x106001 HpkImp
Initial CP0 22 value : 0xe30f3406
node [00000000, 10000000: RAM]
node [20000000, 10000000: RAM]
mips_counter_frequency = 202000000 from Calibration, = 202500000 from header(CPU_MHz/2)
Currently Known Commands:
Kernel message logging via serial can be enabled by connecting the disk to PC and using the bootpage.x86 binary to add "dsscon=true console=0,115200" flags to the bootpage parameters.
Since my USB-SATA adapter was located at /dev/sdb, and the TiVo's bootpage was currently set to "root=/dev/sda4" I issued the following command:
bootpage.x86 -P "root=/dev/sda4 dsscon=true console=0,115200" -C /dev/sdb