Honeywell ACS-8 Installation Instructions Manual page 54

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When two communication modules are to be fitted, install and connect the first module as described above.
Install the second module directly above the first module.
Connect the two together using the attached cable between BU2 of the lower module and BU1 of the upper
one. (See the diagram below.)
Caution! Malfunction possible!
Even lines that are not used, must be terminated!
Installation Instructions / ACS-8