The Type 2 computer prop also known as the bio-computer, made its first appearance in the episode "Miri." That episode required a need to show that there were portable computing units and other instrumentation that could be transported down to the surface of a planet. The type 2 prop was one of the instruments created to fulfill that purpose in the episode. It went on to appear in many episodes in all three seasons. Although the unit was modified and repainted; there appears to have been only one unit made which was a hero model (with working lights and switches).

A good view of the type 2 in its original configuration from the episode "Miri"
The type 2 computer prop was a blue-grey unit with an angled front and a flat top. It sat on top of an unobtrusive black base that appears to be permanently attached to the bottom of the unit. On the front angled surface there was a black panel which had 10 buttons and 10 corresponding red lights on it. The buttons themselves were individually labeled 0 to 9 from left to right. Each button when depressed activated the corresponding red light above it. There was also a black panel on the top of the type 2. On the left of the top panel there was button and a grouping of 4 lights. The button was the same style button that we see on the captain's chair and at each of the various intercom stations. It is a large white button with a black bezel. When pushed this button activated the four lights that would blink in a random pattern. At times it would appear that the lights moved in a specific pattern but they did not. They were just flashing lights that seemed to create random patterns as the differing rates of the flashing would match-up. The lights were red, yellow, green, and white. The top configuration of the type 2 changed in the first season. In the earliest episodes on the top right side there appears to be a simple black hemisphere but this was changed into a light by the end of the first season. Specifically between the episodes "Tomorrow is Yesterday" when it was painted a darker color with the black hemisphere on the top right; to the episode "Devil in the Dark" when it had been modified with the light on the right side of the top and it was painted a lighter "camera friendly" light blue/grey. It remained in this configuration with no modifications for the remainder of the series.
The other button on the top near the center was a small red momentary button. On the early version of this prop with the black hemisphere we never saw this button have any function. After the modification this button activated a larger single flashing light on the right side of the top. Seen most notably in the episode "Operation - Annihilate!" when Kirk realizes that the creatures could be destroyed by light. The only other feature on the top was a data cartridge tape slot on the right side.

”Tomorrow is Yesterday” was the last appearance of the original version before the type 2 was modified

The moment with Kirk realizes that perhaps light will destroy the creatures from “Operation - Annihilate!” Here we see the modified version of the Type 2
Perhaps the most memorable moment we remember of the type 2 is from the scene in “Tomorrow is Yesterday” when in the Captains Quarters it became too affectionate towards Kirk. During the second season it was seen in various places around the ship but typically it was seen in McCoy's lab. It was also featured prominently in the episode "The Omega Glory" along with several other props (reminiscent of the “Miri” episode) on the planet surface as McCoy tried to isolate why the landing party had immunity on the surface.

Here is a great view from “The Omega Glory” of the type 2 clearly showing the modified light on the top right side.

Notice McCoy using the tape slot in this scene from “The Omega Glory”
The type 2 also had a seldom used feature which was a curious rectangular opening on the right side end of this prop. Apparently its purpose was for samples or Petri dishes to be slid into this area for analysis. The slot was also used as data tape storage but this function is only seen in the one episode "Paradise Syndrome." We saw this unit in a few more third season episodes and it made its last appearance in the final episode "Turnabout Intruder."
In April 1977, in the Star Trek Poster Book (if you are old enough to remember those publications), there appeared an article entitled "The Medical Technology of Star Trek" written by Geoffrey Mandel. In the article he refers to this computer as the "Cat Brain" computer. The "Cat Brain" moniker used to describe the type 2 computer can be originally attributed to Mr. James Blish. Mr. Blish wrote the adaptations of the Star Trek episodes into a series of books but he was also a trained biologist. In present-day the term “Cat Brain” when referring to computers indicates a neural network capable of exceeding the processing power of a feline cortex. In the novelization of the “Miri” story; Mr. Blish writes that at McCoy's requests that they get a computer down to the planet to help him with isolating and identifying the disease, Kirk instructs Lieutenant Farrell:
"Ship us down the biggest portable bio-computer--the cat-brain job."
Since Mr. Blish in many cases used early versions of the scripts for the basis of his adaptations; it is not know whether he added the term “cat-brain” on his own or if it was an element that was not used in the aired version of the episode. In any case it was never a part of the final filmed version of “Miri.”

From the Star Trek Poster Book

Devil in the Dark giving us a good view of the side opening. This is the first episode showing the modified type 2 computer

”The Paradise Syndrome” showing the side slot being used for data tape storage

The type 2 was seen when Spock was interrogating Nomad in “The Changeling”

”The Trouble with Tribbles” shows the type 2 where it was often seen; in McCoy’s lab

There is nothing like having your own personal type 2. A Fan made type 2 (built by the author of this series)
The type 2 computer appeared in the following episodes:
12 Miri
21 Tomorrow is Yesterday
26 Devil in the Dark (in the mining office)
29 Operation - Annihilate!
37 The Changeling
40 The Deadly Years
42 The Trouble with Tribbles
44 The Immunity Syndrome
47 Obsession
50 By Any Other Name
51 Return to Tomorrow
54 The Omega Glory
58 The Paradise Syndrome
57 Elaan of Troyius
64 The Tholian Web
68 Wink of an Eye
79 Turnabout Intruder
Next up the Type 3 (http://theomegasecto...rek-tos-part-3/).













In the serial it is seen with a dark finish, sitting on a shelf in the Martian's rocket. On "Trek" it's been repainted silvery grey and has an additional base underneath and cradling the sides. But with it's several rows of pushbuttons, round top center'grill' and central slide paddle it's a distinctive looking prop that I don't recall ever seeing in any other production.