This is post number #1 in a series on how to graph temperature in datacenter from scratch. This is an easy and suitable building project for almost all age groups, competence levels and even budgets. Parts 2 and 3 are published 24. and 26. April, respectively.
Machine room temperatures are often measured using built-in thermal sensors in computers. It is relatively easy to use SNMP to get data from CPU thermal diodes, inlet and outlet air flow temperatures, and then integrate them into some monitoring system with alerts. But the big picture can’t be conveyed using sensor points inside the computers. Design and implementation of a machine room should really include ambient temperature measurements to determine air flow and cooling system supply air temperatures.
Manufacturers tend to have solutions that are prohibitively expensive for bulk use (“starting at $325″), are big (size of a cigarette pack or more) and unwieldy (require separate cabling and electricity supply per unit). Enter summer 1989 and Dallas Semiconductor’s (DS) 1-Wire network (also known as MicroLAN™ or µLAN). This is a low-current, low-voltage bus which requires, at minimum, two conductors for data and power, simplifying cabling, design, and implementation. By 1990, the 1-Wire network protocol had matured and DS introduced first stainless-steel packaged, rugged battery-like memory devices, readable by contact to a reader connected to 1-Wire network. Small, inexpensive, TO92-packaged (small transistor) 1-Wire temperature and humidity sensors have been available at least since the early 1990′s.
1-Wire can use a multitude of topologies, but reliable networks are easiest implemented using bus topology, like in old coaxial ethernet networks. Star topology is not recommended unless 1-Wire switches are used. The term “1-Wire” is a bit misleading, because the network requires at least two conductors: one for data and operating power supply (for so-called parasitic powered devices) and another for signal/power ground. Common reference level (GND, ground) is required, just as with regular RS232 serial port which requires at least TX, RX and GND. Most 1-Wire attached devices can also be externally powered. They then have at least one more conductor for separate operating power supply (usually between 2.5V and 5.5V) and possibly, but not necessarily, a fourth conductor for additional ground lead. Usually signal and power ground can be common.
In other words, 1-Wire networks use either two or three conductors. The parasitic powered networks (two conductors, data and data ground) have stricter limitations than externally powered network with regards to network size and amount of devices that can be attached to the network. Externally powered networks can have lengths of up to 300m and contain tens of sensors. There is probably some upper limit, but DS writes in their application notes that the amount of sensors is virtually unlimited, because every sensor has unique 64-bit ID code.
Voltages and currents used in 1-Wire networks are very small. The devices have idle power requirements of less than 1000 nA and active power is typically less than 1.5 mA. Voltage swing is from -0.8V to +2.2V (minimum for externally powered) or +3.0V (minimum for parasitic powered devices). Suffice to say, for long network runs it is advisable to use good quality, low capacitance (<50pF/m) and low resistance twisted-pair cable and do the connectors in the cable in a professional manner. With little practice and effort, it is easy to build a reliable network a hundred meters long with 15 or more externally powered 1-Wire devices.