Bus Connections & Topologies; Biasing; Figure 1: Eia(Rs)485 Bus Connection Arrangements - GE MiCOM P40 Agile Technical Manual

P44x/EN SC/Hb6
(SC) 12-8
1.2.1.3 Bus connections & topologies
The EIA(RS)485 standard requires that each device be directly connected to the physical
cable that is the communications bus. Stubs and tees are expressly forbidden, as are star
topologies. Loop bus topologies are not part of the EIA(RS)485 standard and are forbidden
by it.
Two-core screened cable is recommended. The specification of the cable will be dependent
on the application, although a multi-strand 0.5 mm
cable length must not exceed 1000m. The screen must be continuous and connected to
ground at one end, normally at the master connection point; it is important to avoid
circulating currents, especially when the cable runs between buildings, for both safety and
noise reasons.
This product does not provide a signal ground connection. If a signal ground connection is
present in the bus cable then it must be ignored, although it must have continuity for the
benefit of other devices connected to the bus. At no stage must the signal ground be
connected to the cables screen or to the product's chassis. This is for both safety and noise
reasons.
1.2.1.4

Biasing

It may also be necessary to bias the signal wires to prevent jabber. Jabber occurs when the
signal level has an indeterminate state because the bus is not being actively driven. This can
occur when all the slaves are in receive mode and the master is slow to turn from receive
mode to transmit mode. This may be because the master purposefully waits in receive
mode, or even in a high impedance state, until it has something to transmit. Jabber causes
the receiving device(s) to miss the first bits of the first character in the packet, which results
in the slave rejecting the message and consequentially not responding. Symptoms of this are
poor response times (due to retries), increasing message error counters, erratic
communications, and even a complete failure to communicate.
Biasing requires that the signal lines be weakly pulled to a defined voltage level of about 1 V.
There should only be one bias point on the bus, which is best situated at the master
connection point. The DC source used for the bias must be clean; otherwise noise will be
injected.
Note:
Figure 1: EIA(RS)485 Bus connection arrangements
Some devices may (optionally) be able to provide the bus bias, in which case external
components will not be required.
SCADA Communications
MiCOM P40 Agile P442, P444
2
per core is normally adequate. Total