Detcon 6400-N4X - Manual (Page 35)

1600/6400-N4X
1600/6400-N4X- Instruction Manual Rev. 1.1 Page 31 of 38
other, data errors may occur. This is even more likely to occur when the distance between ground references
is large. A good way around this issue is to install an isolated repeater into the data bus to isolate the grounds
from each other, thus enabling the bus to use only one of the two references.
Isolated Repeaters
Repeaters can play many rolls in the implementation of an RS-485 data bus. Repeaters can: 1) Extend the
distance if needed to go further than 4000 feet. 2) Allow for the addition of more devices to the bus. 3)
Increase signal strength and integrity. 4) Solve grounding problems and solve some of the problems that occur
when an incorrect wiring scheme is implemented.
A repeater in its basic form consists of two transceivers working together. One transceiver is connected to the
main data bus and the other transceiver connects to the remainder of the devices as it creates a new and
separate data bus. Figure 34 depicts a data bus that implements an undesirable branched wiring scheme.
Depending on the length of the taps of slave 1 and slave 2, they may or may not communicate properly back to
the master. Long lengths of cable to slaves 1 and 2 create four ends to the cable instead of two, which cannot
be balanced with the two terminating resistors on the extreme ends. Without a balanced bus, the long taps of
slave 1 & 2 will introduce reflections to the signal that can lead to problematic operation in the field. These
problems can be very hard to diagnose, much less isolate and fix.
Figure 34 Unbalanced Data Bus
The addition of 2 repeaters (Figure 35) can solve this problem. The repeaters look like short stubs to the main
bus, and at the same time they create 2 new buses that have all the same characteristics as the main bus. Each
new bus could have up to 32 UL’s connected to it. Each leg must have termination resistors to balance the
new data bus.
Figure 35 Data Bus using two repeaters
Be sure to check the equipment being installed. Some manufacturers include a built-in terminating resistor
that can be selected by a jumper or dipswitch.
Detcon 600 and 700 series gas sensors and DA1 modules do not include this resistor, and therefore, if one of
these devices is the last on the RS-485 bus then a discrete 120 ohm resistor should be placed across the “A”
and “B” terminals on the connector board.