BUS Wiring Standards for Video Intercom Systems in Italian Condominiums

The 2-wire BUS topology is the dominant wiring standard for video intercom installations in Italian multi-apartment buildings. Unlike earlier 4-wire or 6-wire analogue configurations, the 2-wire BUS carries both power and communication signal on a single non-polarised cable pair. This characteristic simplified retrofit installations in existing concrete-frame condominiums, where pulling new conduit runs is often impractical without approval from the full condominium assembly.

How 2-Wire BUS Works

In a 2-wire BUS system, all devices — outdoor door stations, indoor monitors, stairwell call units, and power supplies — connect in parallel along the same cable pair. The BUS operates at 27V DC, with the power supply providing both the operating voltage for connected devices and the carrier for digital addressing signals. Because the conductors are non-polarised, installers do not need to track positive and negative orientation, which reduces wiring errors during initial fitting and component replacement.

Bticino's MyHOME BUS system, one of the two most widely deployed in Italy alongside Urmet, specifies that devices connect in a bus topology rather than a star topology. Each indoor monitor and each outdoor panel has a configurable address, typically set via physical DIP switches on the circuit board or, in newer units, through a push-button auto-addressing sequence initiated at commissioning.

Power Supply Sizing

The power supply is the most critical component in a correctly specified 2-wire BUS installation. Standard units for residential condominiums provide 600 mA at 27V DC, which translates to approximately 16 watts of continuous bus capacity. Under typical load conditions — monitors in standby, door station in idle — each apartment draws between 25 and 40 milliamps. A 600 mA supply therefore supports 15 to 24 devices in standby mode.

Active video call conditions are more demanding. When an outdoor station is streaming video to an indoor monitor, combined current draw for that pair rises to approximately 120 mA. Buildings with more than 8 apartments on a single riser typically require a second power supply placed at the midpoint of the riser, or a higher-output 1200 mA supply at the base.

A 600 mA power supply at 27V DC typically covers 8 to 12 apartments in an Italian residential riser, depending on the number of simultaneous active calls anticipated.

Cable Specifications and Riser Design

The BUS cable itself is an unscreened twisted pair, chosen to reduce capacitive coupling between adjacent conductors within the same conduit. Italian installation practice follows CEI 64-8 for low-voltage wiring, with the intercom BUS treated as a Category 1 low-voltage signal circuit. Minimum conductor cross-section is 0.5 mm² for short risers up to 50 metres. For risers between 50 and 150 metres, 0.75 mm² or 1.0 mm² is specified to limit resistive voltage drop below 1.5V over the full riser length. Buildings taller than 15 floors or with long horizontal distribution runs should consider 1.5 mm² conductors and an intermediate power supply.

Cable routing follows the vertical riser from the main technical room on the ground floor upward to each floor junction box. Horizontal branches from the junction box to individual apartments are typically kept under 15 metres in length. The combination of a common vertical riser and short horizontal spurs creates the characteristic "fishbone" distribution pattern seen in Italian building drawings for this type of system.

Device Addressing in Multi-Apartment Installations

Correct addressing is where most commissioning errors occur in field installations. Each outdoor door station must be assigned a building code and a staircase or entrance code, while each indoor monitor receives a flat code. When a visitor presses the button for apartment 4, the outdoor station broadcasts a call telegram on the BUS; only the monitor addressed as flat 4 activates. All other monitors remain in standby.

In buildings with multiple entrances sharing a common BUS — a configuration found in palazzo-style developments with courtyard access — the staircase code distinguishes which outdoor station initiated the call. Without this second layer of addressing, pressing the button at the courtyard gate would ring every monitor in the building rather than only the apartment selected.

Bticino MyHOME and AVE V44 EASY both support address programming through a front-panel sequence that does not require opening the unit or accessing internal switches, which simplifies tenant turnover reconfiguration. Older Urmet and Comelit units from before 2010 typically require physical switch changes inside the monitor housing.

Video Signal on the BUS

Adding video to a 2-wire BUS requires modulating the video signal onto the same pair that carries power and audio. Italian manufacturers solved this using frequency-division multiplexing: power sits at DC, audio occupies the range around 100 kHz to 300 kHz, and video is modulated above 1 MHz. The outdoor camera transmits a continuous video carrier when activated; the indoor monitor demodulates and displays it on a 4.3-inch or 7-inch LCD. This is why video intercoms require a twisted-pair cable with low capacitance — high capacitance attenuates the high-frequency video carrier over riser lengths greater than 100 metres.

Maintenance and Fault Isolation

The parallel bus topology has one known disadvantage: a device with a shorted internal component can load the entire bus, causing all devices to malfunction or the power supply to switch into overload protection. Diagnosing the fault requires disconnecting devices one by one from the bus until current draw returns to normal. Buildings with more than 20 devices benefit from installing bus isolators at each floor junction, which allow a single floor's branch to be disconnected without interrupting service to the rest of the building.

Bticino supplies bus isolator modules for this purpose. During planned maintenance or full-building rewiring, each floor branch can be isolated at the junction, work carried out, and then reconnected without requiring residents above and below to lose intercom access simultaneously.

Compatibility Between Manufacturers

Italian 2-wire BUS systems from different manufacturers are not cross-compatible at the protocol level, despite using the same physical wiring approach. A Bticino outdoor panel cannot address a Urmet indoor monitor, and vice versa. Mixed-manufacturer installations arise only where gateway adapters or IP conversion units sit between the two systems, effectively breaking the BUS into separate segments.

When specifying a replacement for an existing system, verifying the manufacturer and system generation before ordering is therefore essential. In practice, most Italian installers maintain relationships with one or two manufacturer representatives and source complete systems from a single vendor to avoid compatibility issues during commissioning.