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RS485 Interface Board, Laureate RS485 RJ45 interface board Frequently Asked Technical Questions
RS485 is the preferred serial communications standard for long distance industrial communications. It uses balanced differential signals for send and receive. This provides high noise immunity and allows distances up to 4,000 ft (1200 meters). The older RS232 standard uses single-ended signals, is more sensitive to noise, and is only rated for distances up to 50 ft (15 meters). RS485 is specified for multi-point communicates where multiples devices have their own address on the same communication line, while RS232 is only specified for point-to-point communications.
Laureate RS485 boards are available in versions with dual RJ11 jacks (ordering option 2, board P/N L485) and with dual RJ485 jacks (ordering option 4, board P/N LMOD). Both boards support the Modbus protocol and Laurel’s Custom ASCII protocol, as well as half-duplex or full-duplex RS485 communications. The board with dual RJ11 jacks is the more popular since is can use commercial straight-through, non-reversing, 6-wire data cables with RJ11 connectors. Note that these cables are different from4-wire telephone cables or crossover cables.
Use Laurel’s cable CBL06, as described in https://www.laurels.com/cables.php. This cable provides a USB type A connector for the PC end and an RJ11 connector for the meter end. It works in 3-wire half duplex mode, where red & black = GND, green = ATX/ARX, white = BTX/BRX. It incorporates an FTDI RS485-to-USB converter chip which is compatible with all versions of Windows.
Use Laurel’s 6-wire data cables CBL03-1 (1 ft) and CBL03-7 (7 ft) to interconnect meters if these lengths fit your application. These cables allow the dual RJ11 jacks of the RS485 option board of Laureate meters (digital interface ordering option 2) to be daisy chained and to be addressed digitally with no need for a hub. RJ11 jacks support half-duplex or full-duplex communications.
The two jacks are wired in parallel by circuit traces on the circuit board. The two jacks are for RS485 cable in and RS485 cable out. By using two jacks, up to 31 L485 boards can be connected in parallel for daisy chaining using readily available CBL03 data cables with no need for a hub.
Both will work with RJ11 connectors and CBL03 cables. A meter will a thermocouple input and set for Modbus RTU needs to set to full duplex even if the wiring and the receiving unit are for half duplex. In all other cases, half duplex and full duplex will work equally well. Half duplex has the advantage of only using three wires: ARX/ATX, BRX/BTX and GND. With Modbus RTU, sending and receiving are never simultaneous, so half duplex is as fast as full duplex.
A challenge is that Laureate meters use RJ11 jacks, while Laureate LT series transmitters use screw-clamp connectors for three discrete wires. The solution is provided by the Laureate DIN rail mounted breakout board, which provides two RJ11 jacks with six screw clamp terminals all wired in parallel. With this boards, a CBL03 lines with RJ11 connectors can branch out to support an RS485 line consisting of three discrete wires. Not more than one breakout board is required per RS485 line.
Laureate meters are modular with a middle slot reserved for a communication board, as illustrated in our Laureates Overview web page. Boards can be ordered installed with a new meter, and they can also be purchased separately later and be simply plugged in. The presence of a new board and the type of board are automatically sensed by the meter’s firmware or by Instrument Setup (IS) software. If you change boards, also change the model number on the meter label.
Laureate meter and LT series transmitters support the Modbus RTU protocol and Laurel’s simpler ASCII character based Custom ASCII protocol. Both protocols (or communication languages) are built into the instrument’s firmware and are user selectable.
A single analog input meter or transmitter can stream readings to a PC at up to 60 readings per second and the Custom ASCII protocol set to continuous (or streaming) model. If multiple analog input meters or transmitters are digitally addressed in command mode on an RS485 line using the Modbus RTU protocol or the Custom ASCII protocol, the rate becomes about 2 readings per second. Read rates vary with counters, since they are normally controlled by the user entry Gate Time.
When a single RS485 meter or counter is connected to a PC via a cable CBL06, set the PC to 9600 baud. Set the PC to N81 for the Custom ASCII protocol, or to N82 or E81 for the Modbus RTU protocol. To set up communications for your meter or counter, see the Serial Communications section of its user manual. First set jumpers for your type of communications. Then use front panel programming to set the baud rate, protocol, digital address, half or full duplex, and other available parameters. Setup is the same when multiple meters are interconnected via cables CBL03, except that all meters now need to be set to a different digital address via their front panel.
If the PC is connected to the multiple Laureate instruments on an RS485 bus via an Ethernet-to-RS-485 gateway board or a USB-to-RS485 gateway board, use Laurel’s free XLOG2 datalogging software. That software is described in our web page at https://www.laurels.com/datalogging-software.php and user manual at https://www.laurels.com/downloadfiles/datalogging-manual.pdf.
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