Description
Our interface board with dual RJ11 jacks allows multiple DPM-3 digital panel meters, to be connected to the same RS485 line. The two RJ11 jacks (or connectors) are wired in parallel. One of the jacks can be connected to the USB port of a PC by using our RS485-to-USB converter cable for data I/O or for programming by a PC running our Instrument Setup (IS) software. Having two RJ11 jacks wired in parallel allows multiple instruments to be daisy-chained using readily available 6-wire data cables or patch cords with RJ11 plugs. In the absence of a computer, a single DPM-3 digital panel meter with an RS485 board can drive multiple 6-digit remote displays.
Ideal complements to the RS485 interface board are Ethernet-to-RS485 converter board, Ethernet-to-RS485 gateway board, which connects a meter network to an Ethernet LAN, and the USB-to-RS485 converter board which connects a meter network to a host PC via a standard USB cable. Both gateway boards provide an RJ11 interface to an RS485 network, which can consist of multiple DPM-3 meters that are easily daisy-chained.
Operation can be configured for full duplex or half-duplex operation using jumpers on the RS485 board. Transmission distances up to 4,000 ft (1200 meters) are possible. In case of long line lengths, a jumper can be plugged in the RS485 board at the end of the line to insert a 120-ohm termination resistor to minimize signal reflections. The DPM-3 meter, should be programmed for full duplex operation, even if the RS485 board is jumpered for half duplex.
Two serial protocols are user-selectable for RS485: Modbus RTU and Custom ASCII. For detailed information on our protocols, please see our DPM-3 Serial Communication Manual.
- The Modbus RTU protocol is an international standard and allows DPM-3 Meters to share the same RS485 data line with other devices. It supports up to 247 digital addresses. Use of more than 31 DPM-3 RS485 Modbus boards requires repeaters for impedance reasons. Modbus RTU works in a master-slave (or query-response) mode, and its use requires an external Modbus master, which is typically a PC.
- Custom ASCII protocol is the simpler of the two protocols and uses ASCII strings. It is recommended if the only devices on the RS485 data line are DPM-3 meters. It supports up to 31 digital addresses. It can be set to a command mode under control of an external master, or to a continuous streaming mode where data is output at specified intervals at rates up to 60 per second.
The DPM-3-OPT-RS485 interface board is similar to DPM-3-OPT-RS232 interface board, except that it provides dual RJ45 connectors instead of RJ11 connectors. That board is not supportedby readily available data cables. Therefore, use the DPM-3-OPT-RS232 board with dual RJ11 connectors as described in this web page, unless design specifications call our RJ45 connectors.