EMS bus wiring

This page provides an overview of connecting a device to the EMS bus.

Using the service jack

If your boiler or heatpump has a service jack, just connect the Gateway with the EMS service cable. The service jack has pins for both the EMS bus and an additional power supply for peripherals. You do not need to add an external power supply in this case. You can get a very high quality EMS service cable from the webshop, and one is also included in the KIT products. This cable has the proper wire diameter inside which is necessary for also powering the Gateway. Furthermore the cable has extended plugs, so it will fit into the recessed holes of certain boiler als also into the recessed hole of the E32 V2. In some cases the boiler or heat pump lid or cover cannot close when the EMS service cable is inserted. You need to mount the EMS service angled adapter in order to close it again.

Warning

Do not use cheap audio cables for connecting a Gateway to the service jack of the boiler/heatpump. Most of them have very thin wires inside, this may cause lots of interference and it is a possible fire risk. The EMS service cable from the webshop is a high quality cable with thicker wires inside.

Splitting service jack power

Sometimes there is already a device plugged in to the EMS service jack. In theory you could split the wires. This is not recommended. If it is not possible to use the EMS service jack, use the EMS screw terminal instead of splitting a EMS service jack cable. Also the ground of the DC power supply pin is common with the EMS- pin, this can cause issues when you connect two devices on the same cable.

Using the EMS screw terminal

If you want to use the screw terminal on the Gateway, you need to connect a cable with two wires between the screw terminal of the Gateway to the EMS terminal inside the boiler (usually an orange or white connector). If there is already something attached to the EMS terminal in the boiler / heat pump, you can connect the Gateway in parallel on that connector. Do not connect other EMS bus participants to the screw terminal of the Gateway.

According to Bosch, the total length of cable between all EMS participants on the same bus is:

  • 100m for 0,50 mm2 wire.

  • 300m for 1,50 mm2 wire.

Furthermore there are some additional requirements to adhere to:

  • If several EMS bus participants are installed, they must be have a minimum distance of 100 mm between each other.

  • If several EMS bus participants are installed, they must be connected in parallel or in a star star-shaped topology. Do not create a ring topology.

  • With external inductive influences (e.g. from photovoltaic systems) use shielded cable. Ground the shielding on only one side of the housing.

So in a practical situation you have at least several dozens of meters available.

It does not matter here if it is solid or stranded wire.

For connecting a Gateway, if you run up to several meters of cable length, an unshielded cable with 2x0,5 mm2 conductors is more than sufficient.

Alternatively you can f.i. use a piece of left over Ethernet cable where you combine two conductors for each EMS pin. If the cable has shielding, only connect the shielding to the boiler/heat pump side. Do not connect any shielding to the EMS bus pins or other parts of the Gateway.

EMS bus wire polarity

Although most EMS terminals have a EMS+ and EMS ‘ground’ pin (even when not marked as such), the EMS bus devices are usually not polarity sensitive. This means it does not matter which of the two EMS wires you connect to which of the two screw terminal pins.

The EMS Gateways have an internal correction circuit so both ways will work. However, some devices like the RC310 can give issues if wired incorrectly. If the screen does not turn on, reverse the wires on this device.

Warning

Never connect both the EMS screw terminal together with the EMS service cable. You may short circuit the EMS bus by doing so.