The RTC Multi-link is a hardware/software networking solution for the Commodore 8-bit link of computers, and compatible with the following machines:

Commodore 64 Breadbin (long board)
Commodore 64c Slim case (short board)
Commodore SX-64 (Executive/portable)
Commodore 128
Individual Computers Reloaded V1
Individual Computers Reloaded V2
Gideon's Ultimate 64 (all versions)

This product/item is the Master Control cartridge required to control the network.

It is offered here as a separate product for those people that own original Multi-Link Satellite cartridges, but are missing the master control cartridge.  For those people in this situation, all they will need is this cartridge and the software and manual (that are available through this site).

To use this item, you will also need to own one or more satellite Multi-Link cartridges.  Take note that the maximum number of networked machines on this system is 6 + master.  

When setting up your network hardware, this "grey" cased cartridge is the "Master" control cartridge must be inserted into the computer that will function as the master control machine.  This must be a Commodore 64, and cannot be a 128.  Your remaining satellite cartridges and/or Multi-Link128 cartridge will complete your hardware deployment.  Each satellite cartridge plugs into the cartridge port of the machine to become part of the network.  The ribbon cables connect the grey master cartridge to the satellite cartridges.  The order in which they are connected is not important.  The first cable plugs into the grey master cartridge, and the other end of it connects to the first computer in the network.  Then next satellite is linked daisy chained in.  

Follow the instruction manual available for download for complete network setup and configuration.