ABB AC 800M, User Manual

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Adding field devices to the MBP network Section 2 Transmission Technology
38 3BDS009029R5001 B
Adding field devices to the MBP network
PROFIBUS PA devices are mostly connected to the network by using multibarries
or T-junctions. The resulting spur has to be connected directly to the connector
block inside the device. Sometimes also M12 plugs are used to connect the device to
the network (see also
Bus connector on page 67).
When using MBP technology, incorrect connection of a field device (for example
polarity reversal) has no effect on the functionality of the bus as these devices are
usually fitted with an automatic polarity detection function.
The Manchester signal used by fieldbus is an alternating voltage that changes
polarity once or twice per bit. In self-powered networks only this alternating voltage
exists.
In powered networks the alternating voltage is superimposed onto the DC voltage
being used to power the devices. In either case, the fieldbus receive circuits look at
only the alternating voltage. Positive voltage swings have one meaning, negative
swings have the opposite meaning. Therefore, the fieldbus signal is polarized. Field
devices must be connected so that they all see the signal in correct polarity. If a field
device is connected “backwards” it will see an inverted version of the alternating
voltage and won’t be able to communicate. However, there are non-polarized field
devices available. This field devices can be connected in either polarity across the
network. The non-polarized devices are invariably network-powered, which
provides a clue as to how they can work with either polarity.
If a fieldbus network is build up to accept all possible devices, the signal polarity
must be taken into account. All of the (+) terminals must be connected to each other.
Similarly, all of the (-) terminals must be connected to each other. Color-coded wire
makes this relatively easy. Polarized devices should always be marked or have
keyed connectors. Non-polarized devices might not be marked. It may be possible to
select devices with the intention of never having to consider polarity. But it may be
safer to build polarity into the network at the outset and accept as a bonus any
devices that can be blindly connected. This approach also lets to expand the network
with polarized devices if needed later on. Bus powered field devices may also be
When selecting the type of line, make certain that for Ex applications the
requirements in terms of insulation voltage and single conductor insulation of EN
60079-14 and the characteristic values in accordance with the FISCO model are
maintained.