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4.8 RS-485 Installation
The RS-485 (EIA-485) standard specifies the electrical characteristics for a digital
communication link allowing communication between multiple devices on a single link.
The RS-485 uses two wires, A+ and B-, and works on the voltage difference between
them. If the voltage difference is positive, then that is a "1" if negative then that is a "0".
Connections: Wire terminals A+ to A+ to A+ etc., and B- to B- to B- etc.
End-Of-Line Termination Resistance: The terminator on each end of the RS485 loop is
designed to match the electrical impedance characteristic of the twisted pair loop, and
will prevent signal echoes from corrupting the data on the line. The terminator should be
enabled on BOTH ends of the RS485 loop. Short and medium length modbus/485 loops
can operate without the terminating resistor. Longer runs may require the terminating
resistors. But adding terminator dramatically increases power consumption. All QEL
equipment supplies the end-of-line resistor on the circuit card. It is enabled or disabled
with a shunt jumper. (See relevant Installation Drawings for information).
Q-Controller supplies this resistor on the main board, and it is chosen using a jumper at
J2, J5, J7, J8 and J9.
Factory default setting is disabled terminator.
Distances: The RS-485 standard allows up to 1300 meters (4000 feet) of line length. It is
best to avoid lines of this length if at all possible.
Stubs: Short lengths of cable from the main cable over to a device are called Stubs.
When the Baud rate (communication bit rate) is low – e.g. 2400 baud, then it is often
possible to use short lengths of a few inches without seriously impairing the signal
integrity, especially when overall distances are relatively short; however, this is taking a
chance on garbling your signals and is not recommended.
Cable Shields: Cable shields are aluminum and so only 'proof' against electrical fields,
not against magnetic fields. The twist in the pair is to reduce the effects of magnetic
fields. Take care not to run cable close to magnetic sources. Iron conduit is a good shield
for both electrical and magnetic fields.
Shield Grounding: There are certain things to keep in mind for the shield.
• The shield must be grounded otherwise it can make the situation worse.
• Ground the shield at only one end to prevent ground loops.
Q-Controller System Operation Manual
86550-001-000 Rev B
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