When an RTDS Simulator consists of three or more racks, a special purpose hub – referred to as a Global Bus Hub (GBH) – is required to exchange synchronization signals between GTWIF cards.

An RTDS Simulator consisting of a single rack operates independently and a two-rack RTDS Simulator can operate by making a direct connection between the two GTWIF cards. For RTDS Simulators with 3 racks or more, each GTWIF connects to the GBH using a fibre optic Tx/Rx cable. The GBH is accessible from the rear of the cubicle. GBH communication is separate from the Ethernet communication used to exchange information between GTWIF and the host workstation.

The Global Bus Hub is often located in a special auxiliary cubicle along with the IRC Switch and Ethernet Switch.

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The GBH enclosure is installed in the rear of one of the RTDS cubicles and may be equipped with one, two or three hub cards. Each hub card permits connection of up to 10 GTWIF or WIF cards, for a total of 30 synchronized racks. It is also possible to install 2 GBH enclosures, each with up to 3 hub cards, so that an RTDS Simulator may consist of up to 60 synchronized racks.