Figure 9.2.1 EIA RS-485 bus connection arrangements
9.2.1.3 Bus Connections & Topologies
The EIA RS-485 standard requires that each device is directly connected to the physical cable that
is the communications bus. Stubs and tees are expressly forbidden, such as star topologies. Loop
bus topologies are not part of the EIA RS-485 standard and are forbidden by it also.
Two-core screened cable is recommended. The specification of the cable will be dependent on the
application, although a multi-strand 0.5mm
2
per core is normally adequate. Total cable length must
not exceed 500m. 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.
9.2.1.4 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 these 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 1V. 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 that some
devices may (optionally) be able to provide the bus bias, in which case external components will
not be required.
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