Mattel, like all other console manufacturers of the day with the exception of Atari who already had a line of 8-bit machines, saw their Intellivision console as a serious contender in the home computing market as well as being a successful games machine. Unfortunately, marketing dreams and reality never really mesh well together and the resulting Entertainment Computer System leaves a lot to be desired particularly when you consider what it was supposed to be competing with.
The system came in 2 bits, 3 if you count the extra PSU required. The main module plugged into the cart socket of the Intellivision and provided an extra whopping 2K of RAM, 16K ROM, a BASIC interpreter, cassette and a printer port and the ability to access the World's Largest on-screen character set after the Philips G7000 (or Magnavox Odyssey 2 for US types). Oh, there were 3 extra voice channels too. The 2nd bit was the surprisingly easy to type on chiclet keyboard that connected via a frankenstein's monster type of connector to 2 sockets under the front flap of the computer module. OK it was just 2 standard joystick plugs connected together but you get the idea :) The 2 ports were to allow 2 extra controllers to be added for 4-way player action, though how many games took advantage of that I don't know.
The BASIC interpreter only allows 4 bytes per command (PRIN instead of PRINT etc) and a handful of additional routines to eg make and cancel noise. They were obviously aiming at the complete novice market since the interpreter actually features syntax highlighting so the commands, modifiers and variables are coloured differently so you can see if you've boobed!
Looking through the manual I get the feeling that the ECS' only real purpose was to allow you to write BASIC games that used the Intellivision character set; pity they tried to use the word 'powerful' in the sales bumf......oops.
Apparently this module 'ere is much less common than the white one that was made for the Intellivision II in the US.....