Some vintage computing and gaming devices came with two interconnected components. They came with two distinct physical elements that combined together to function. Here I will give examples of what I mean :
Roland MPU-401 Units
The original Roland MPU-401 unit housed all its circuitry (microcontroller, RAM and firmware) in a metal box. This box connected via a male to male DB-25 cable to an interface card or cartridge for the computer in question. The unit had DIN connectors for MIDI IN and OUT and was intended to connect to MIDI devices. The interface card (MIF-IPC, MIF-IPC-A) was always a simple bit of circuitry to provide an input and an output port to the MPU-401. Typically, when a Roland MPU-401 is marketed for sale, it will only come with the unit, and not an interface card. While the circuitry in the unit handles all the intelligent MPU-401 commands, without an interface card, a PC has no way to connect to it. Designing a prototype interface board is possible, but not necessarily something that just anybody can do. Recently, there are clones available for the MIF-IPC-A, which is compatible with just about any PC with an ISA slot. However, they are really pricey for a simple card with no custom chips.
You can purchase perfectly functional replicas of the interface board here : https://www.lo-tech.co.uk/
Roland MPU-IPC Line
Roland later released the MPU-IPC, MPU-IPC-T and MPU-IMC. In this case, while there was a combo of an ISA card and an breakout box, this time all the circuitry was placed on the ISA card. The breakout box contained just the physical MIDI ports and some passive components. Usually, when these are advertised, only the breakout box is listed. The box on its own is useless. The card without the box also has no practical purpose unless you are trying to explore the MPU-401 as a programmer. Fortunately, if you have the card, it is feasible to solder together a MIDI OUT port so you can connect your MT-32 or other MIDI device to it. Even implementing a MIDI IN port is feasible with an opto-isolator. The MPU-IPC-T's manual, freely available online, gives the schematic for both it and the MPU-IPC.
The Roland LAPC-I is not useless without its breakout box, the MCB-1. The MCB-1 is useless without its card. However, they were sold separately, whereas for the MPU-IPC packages, card and box came together. The only thing you miss with an MCB-1 is the ability to connect external MIDI modules. The same applies for the IBM Music Feature card and its breakout box, but in IBM's case, the card came with the box. I have read that you can repurpose a common gameport-to-MIDI adapter to substitute for an MCB-1 because they both use a DA-15 connector. This pinout would almost certainly work :
LAPC I DA-15 Connector
+5v - 8, 11
GND - 9, 11
MIDI IN - 14
MIDI OUT - 13
Sound Blaster DA-15 Connector
+5v - 1, 8, 9
GND - 4, 5
MIDI OUT - 12
MIDI IN - 15
Wireless Controllers : Atari Remote Control Wireless Joysticks to Nintendo GameCube WaveBird Controller
Each Atari Wireless Joystick has an antenna jutting out of it and, compared to a wired joystick, a huge base housing the RF circuitry and the battery compartment. The receiver is a black box with a retractable metal antenna that plugs into the joystick ports of the 2600. The 2600's power adapter plugs into the receiver, which then has a cable which channels the power to the console. The range on these controllers was so poor that they were not worth the all the hassle.
Due to the unwieldy nature of Atari's RF solution, for the rest of the 1980s and 1990s, most controllers used Infrared Receiver Technology. This is the same type of technology found in your cable remote. Some controllers had an IR transmitter built into them, which did not add nearly as much weight and bulk (even with batteries) as the RF solutions did. All required a receiver to be plugged into a controller port. Nintendo released a 4-player adapter called the NES Satellite. The Satellite had a base where you could plug in four controllers. It also had a receiver which plugged into both of the NES's controller ports. Similarly, the SNES Super Scope also used a wireless IR receiver to determine the "gun's" position.
The WaveBird controller was the first modern wireless controller. It used RF signals in the 900MHz and 2.4GHz bands and did not require an unobstructed line-of-sight like previous IR controllers. The range was superior to IR controllers, supporting operation 20 feet from the console. It no longer mattered where the player was in relation to the receiver or what was between him and the receiver (within reason). The WaveBird was not substantially larger than the regular wired GameCube controller, unlike the Atari Wireless Joysticks. Unfortunately, the receivers are really small and often times get lost and thus are not included with every WaveBird auction. By the seventh generation, all wireless controllers used Bluetooth technology, with the transmitter/receiver located in the console.
Game Boy Player
The Nintendo Game Boy Player attaches to one of the ports underneath a Nintendo GameCube. It allows you to play Game Boy, Game Boy Color and Game Boy Advance Cartridges on the GameCube and on a TV screen, similar to the Super Game Boy for the SNES. However, while the Super Game Boy contained everything it needed to run inside its cartridge, the Game Boy Player includes a software disc. This disc must be present in the GameCube and must load before you can use the Game Boy Player. The Player screws into the underneath of the GameCube, but the mini-disc and its small case tended to get lost. The GameCube's copy protection must be bypassed to use a backup of the software disc. This is the only official way to play Game Boy Color or Game Boy Advance games on a TV screen.
However, you need not despair anymore if you have the Player and don't have a disc. You can run Game Boy Interface, which can do even a better job than the real disc! Start here : http://retrorgb.com/gameboyinterface.html
Games Designed for a Particular Peripheral : R.O.B.
(I am not going to go through every example of a game that works with only a certain peripheral, but a few special cases come to mind)
Nintendo released R.O.B., the Robotic Operating Buddy, with the NES Deluxe Set back in 1985. R.O.B. came in this set with the pack-in game Gyromite. R.O.B. was also released alone and without a pack-in game. It is not uncommon to find loose R.O.B.s or Gyromite or even Stack-Up cartridges. However, without the special accessories for each game, R.O.B. is useless. Because the Gyromite accessories came with systems, they are more common than the Stack-Up accessories. However, finding complete sets of accessories is also a hit or miss affair. Gyromite has five pieces (two gyros, gyro holder, gyro spinner, controller stand) and Stack Up has ten (five blocks and five stands).
Games Designed for a Particular Peripheral : Miracle Piano Teaching System
The Miracle Piano Teaching System was a peripheral for the NES, SNES and Genesis, and also worked with the PC, Macintosh and Amiga systems. The Miracle Piano was a 49-key MIDI keyboard and came with software either on cartridge or disk. It is enormous as far as peripherals go. It also came with a custom cable to plug into the console's controller port and a foot pedal. On a PC, a pair of MIDI ports would work. All sound would be generated by the keyboard's speakers. The piano itself is the same regardless of the system it was intended for, only the software and cable differs from system to system. Loose carts do appear as well as loose pianos, but the cables tend to get lost. Pinouts for the cables can be found here : http://pianoeducation.org/pnompcab.html