Detcon 1600A-N1P - Manual (Page 35)

1600A-N1P
Appendix A
RS-485 System Integration Wiring
RS-485 is a generic electrical specification only. It does not specify connector types, protocols, or cable type.
It is the responsibility of the installer to provide a correct physical installation for the RS-485 network to
function properly.
RS-485 is very inexpensive and inherently rugged, allowing multiple devices to communicate over a single
twisted pair of wires. Detcon has chosen to implement the Modbus™ protocol using the RS-485 electrical
specification.
RS-485 is a two way half-duplex data bus made up of a transceiver (transmitter and receiver). In its simplest
form, one transceiver is located on each end of a twisted pair cable of up to 4,000 feet maximum. Data can be
sent in either direction, but can only be sent in one direction at a time.
The specification allows for the connection of up to 40 unit loads (UL’s) to the twisted pair. An older
transceiver chip would have represented 1 UL. Connection of more than 40 UL’s to the bus will limit the
drivers and cause attenuation of the signal, thus preventing the system from operating properly.
Since RS-485 allows multiple devices to be connected to the bus, wiring is not as straightforward as it is in a
bus with only 2 devices. The best wiring scheme is the daisy-chain connection from device 1 to device 2 to
device 3 to device n. All devices that tap into the bus should not be at the end of long stubs, branches, or t-
taps. A true daisy-chain has direct wiring between devices with no minor branches.
Detcon equipment contains four wiring terminals for RS-485 connections. Two for A(+) and two for B(-). All
connections should be made directly to these. Figure 3.1 shows an electrical representation of a simple RS-
485 bus with 4 transceiver chips.
Figure 32 RS-485 Bus with 4 transceiver chips
The use of a daisy-chain connection will perform the best in all applications given the bus is properly
terminated. The RS-485 standard specifies the transceiver characteristics only, not the interconnection of the
transceivers themselves. Depending on the environment, distance and other factors it is possible to connect
transceivers using flat cable, twisted pair cable, even PCB traces. However, for the most reliable data transfer
1600A-N1P Instruction Manual Rev. 0.1 Page 31 of 40