Eicon Networks ISDN NT1 Specifikace stáhnout pdf (Strana 49)

Another form of wiring is known as the extended passive bus. In this case the total

maximum distance allowed is greater, up to about 500 metres, but there is a restriction on

where the devices may be located on the main cable.

The maximum length of the drop cable is the same as before (ten metres). The restriction

is that the device nearest the NT1 must be no more than either 25 or 50 metres from the

end of the main cable furthest from the NT1. The smaller distance is for 100Ω cable, the

larger for 200Ω cable.

The cable for the BRI can contain as many as eight conductors. However, four of these

are optional. They are organised in pairs like this.

Notice how there is one pair of conductors which carry data away from the devices toward

the NT1 and another pair which carry data from the NT1 toward the devices.

The red lines show the optional pairs. These are concerned with providing power to

devices connected to the BRI. This is a convenience for an ISDN telephone, that, for

example, has an LCD and LEDs. This therefore needs an external power supply which can

be supplied through its ISDN interface rather than through a separate mains power

adapter.

Power provided by some NT1s

PS2 is a power supply provided by the NT1. Not all NT1s can do this as standard. For

instance, in the UK this is a separate, chargeable feature.

PS3 is a mechanism through which one device can supply power to other devices. This is

very rarely implemented.

There is another power supply known as PS1. This is also known as the "phantom" power

supply. It is found in the voltage difference between the transmit and receive data pairs.

This derives from the network itself and is normally used for emergency situations where

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Eicon Networks ISDN NT1 Specifikace Strana 49