Robert Knox acquired his Type 70 TARDIS from a Nekkistani dealer on Gryben, something which didn't impress the Sixth Doctor when he discovered Knox was a human and not a Time Lord like himself. According to Knox, the Nekkistani had found the TARDIS just abandoned.
It had a working chameleon circuit, having taken on forms like a cabinet, a thorny bush, and a house. The TARDIS could be operated by voice control, but it could also be switched over to manual control by a simple verbal command. This system allowed Knox to also perform mundane jobs like opening the main doors and taking Knox up or down in the metal elevator to different levels within the ship. But while he clearly relied on the voice control system to help him control the TARDIS, Knox was also adept at understanding the controls, as he helped the Sixth Doctor operate the console when the Doctor had problems because the Type 70 was more advanced.
During the Doctor's next meeting with Knox in Washington, 1865 the TARDIS could also run checks on itself and come back quickly with a response. Like other TARDISes, such as the Doctor's, it featured a fast return switch. Despite his experience, the Doctor had problems operating Knox' TARDIS since it was a more advanced model than what he was more accustomed to, prompting Knox to step in to help. At the time Knox boasted that he had 'traded up' and installed the 'mod cons,' like the voice control system and the elevator. The Doctor was taken aback by these inclusions.
The TARDIS displayed its intelligence when Knox was in a bad way due to his dying body and suggested to him that he should head for the sickbay on Level Gamma 7. The ship played the 'Can-Can' which Knox thought to be jolly, though he stopped it when he arrived on the floor in question.
Knox's dying body was being sustained by an Indo he'd found, and he'd promised it that he would take it with him through history but the creature was imprisoned inside his TARDIS in an iron cage that it couldn't escape from. When another creature came after him, the stress got to Knox and so the Doctor was forced to deal with the problem by taking the second creature into himself. The Doctor trapped the second Indo inside the elevator, which was a metal box in itself, and told the ship to never open the elevator ever again.
Unfortunately, Knox had wanted to deal with the Doctor as well and recorded a message for the TARDIS to play when the second Indo was dealt with. The TARDIS would travel to Mercury where the ship would stay trapped for all eternity and the Doctor wouldn't be able to escape. The Doctor was able to re-set the TARDIS's galactic positioning system and return to Washington where he'd set off from though it probably took him a while since his companion Evelyn saw streaks of grey in his hair. (AUDIO: Assassin in the Limelight)