Many Allen & Heath products can have their busses linked so as to expand input/output capability.
We call this Sys-Link™ and it requires the purchase and fitting of special PCBs to each mixer.

The signals are presented on D connectors on the rear of the mixers and linked with regular computer type cables.
When linked, all the busses of one mixer (slave) are connected directly to the corresponding busses on the second (master) mixer.

Different models can be linked. An example is a Wizard 14 linked to a GL2800.
The Wizard auxes 1-6 are linked to the GL2800 auxs 1-6.
The Wizard groups 1-4 are linked to the GL2800 groups 1-4.
The Wizard LR are linked to the GL LR.

The PFL/AFL signals and control are also linked - if you select PFL on a Wizard channel,
you hear it on the GL headphones and see it on the GL master meters.

ML 4000 and ML5000 are not Sys-Link™ compatible. 

The external buss connections on these two models are only for ML Sidecar.

Apart from having different operating levels from Sys-Link, the ML4/5 connections are separate for Audio and logic.
It is the separate logic control for PAFL and mute groups that is the main thing preventing connection to Sys-Link™.

ML3000 is Sys-Link™ compatible and is not sidecar compatible.

Sys-Link™ Compatability Chart is available <here>

System link fitting and application notes are available on the relevant product pages

Contact you dealer or local Allen&Heath distributor for more information on Sys-Link™ compatibility and to order parts.

keywords: Sys-Link SysLink  Master. Slave. D-subminiature Dsubminiature  connector. Expand. ML3 ML4 ML5