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Power Distribution Unit (PDU) Technical Terms and Basic Understanding
PMU-Power Monitoring Unit The product provides the ability to monitor device power levels and convey this information both visually and digitally over a network. We use this board as a means of measuring voltage, current & power being delivered to a mains powered device, displaying this information on an LCD and also conveying this information over an RS232 or RS485 network *BS EN 60950 – Information Technology – Equipment Safety *Low Voltage Directive 73/23/EEC modified by Directive 93/68/EEC [CE Marking] *EMC Directive - 89/336/EEC amended by Directive 91/263/EEC *EMC Directive - 92/31/EEC and Directive 93/68/EEC *RoHS Directive 2002/95/EC *EIA485 RS485 *EIA232 RS232
This board is the heart of all the monitoring calculations and communications, this is needed in all of the PDU’s that require either remote monitoring or at least to display the information on the PDU LCD itself. The PMU samples the mains supply over 4000 times a second to provide true RMS Voltage, Current and kWh to Class II accuracy. (Power kWh direct reading +/- 2% (kWh to BSEN 61036:1997) Class II Cumulative reading). The PMU has the following internally calculated Energy Readings, Voltage (True RMS Reading) Current (True RMS Reading) Kilowatt Hour (Cumulative Reading), Class II Accuracy (Billing Quality). Kilograms of CO2 (Cumulative Reading). Kilo Joules Kilowatts Power Factor BTU’s (British Thermal Units) Also additional readings include Peek Current, Peek Voltage, Apparent Power, and Harmonic Active Power. All Calculations are made using a True Power Factor reading, (Not an Assumed Power Factor); The Power Factor reading is key as this provides the accuracy for the Kilowatt readings and is dependent on the type of equipment drawing power. Most of the above readings can be viewed on a local LCD Display as part of the PDU and also using the IP/SNMP unit if added as an optional extra. The PMU has 2 RJ45 sockets which offer Modbus communication which when set at the factory can either communicate using RS485 or RS232, these are not to be mistaken for Ethernet ports.
RS485 EIA-485 [TIA-485] Balanced (differential) interface; defines the Physical layer, signalling protocol is not defined. RS-485 supports bi-directional, Multi-Point, half-duplex data transmission. RS232 EIA-232 Standardised connection system for connecting a device to the serial port of a computer or terminal.This is the recommended standard of the Electronic Industries Association (EIA) for exchanging information between DTE (such as computers) and DCE (such as modems)
The favoured form of communications is the RS485 version, which gives the user the ability to daisy chain to other PMU boards using standard CAT 5 patch cable up to 32 in total. All these daisy chained PMU boards can then be monitored over the network using relevant software.
When connected to the mains the PMU board will display when first booting up a version number of the firmware and then proceed to display Volts and Amps across the top of the LCD and along the bottom you can either have a static display of certain information of a scrolling display of kWh, kgCO Btu/h, kj/h and Watts Also along the top of the display every few seconds the Modbus address will be displayed as ADR: 1 or ADR: 2 etc. If daisy chaining these PMU boards together then each unit has to have a different address. Modbus is a serial communications protocol published by Modicon in 1979 for use with its programmable logic controllers (PLCs). It has become a de facto standard communications protocol in industry, and is now the most commonly available means of connecting industrial electronic devices. So in theory, if you wish to daisy chain the maximum number of PMU boards together in order to be able to communicate with these then they should be numbered 1 to 32. Modbus can go higher but as our IP/SNMP unit can only talk to a maximum of 32 units then numbering them this way is the most common recommendation. The PMU boards 2 RJ45 sockets can be used as an ‘out’ and ‘in’ when daisy chaining, and as these sockets are exactly the same, either can be used for out and in. If these PDU’s are going to be daisy chained together and the Web Browser is going to be used, it is good practice and recommended that at the last PDU to be daisy chained, in the spare RJ45 socket an RJ45 plug should be inserted linking pins 2 & 6 and 3 & 7 together. This is because the Web Browser talks to the PDU’s in a 2 wire form, and without inserting this RJ45 plug the communication is not looped properly and could slow down the communication to the Web Browser if there are a number of PDU’s daisy chained.
PMU Board additional options Because the PMU board uses a 1 wire form of communication, there are a couple of optional extras which can be added on. 1 wire is a low-cost bidirectional serial communications bus requiring just a single wire to implement. An asynchronous, half-duplex, single-master component bus, 1-Wire minimizes device pin count by using extra on-chip circuitry to enable each 1-Wire component to communicate serially (over a shared connection) with other 1-Wire devices. Compatible serial EEPROMs, A/D converters, temperature sensors, and other devices are available. Electrical features include lo data rates (about 16 kbps, max), flexible voltage and timing requirements, long maximum distances (up to 1,000 feet), and optional parasitic powering of slaves. Each 1-Wire device has a unique 48-bit serial number, so multiple slaves can share the same bus.
