Q4 CONTROLLER II Operation and Maintenance Manual
Q4 Controller II Operation Manual
- 9 -
Dec 02, 2021
85350-101-000 Rev A
4.6 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.
Terminator:
Q4-Controller supplies two end-of-line resistor on the main board, one for sensor
RS-485 port, one for Modbus/BACnet port, they are chosen using a jumper at J3, J4.
J3/4 1-2: Terminator Disabled/Off (Default)
J3/4 2-3: Terminator Enabled/On
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.
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.
If you cut the cable then either ground each section of the shield at that point or connect
the shields together to ground back at an origin point.
RS-485 Driver Replacement:
RS-485 lines in heavy industrial environments are sometimes
subjected to magnetic disturbances causing sufficient inducted power surges to damage the driver
integrated circuit (IC). The IC U5 and U6 have sockets on the circuit card for ease of replacement
in the field.
U5 U6: RS-485 TRANSCEIVER [QEL SKU#: 3200-0044]
