Campbell Scientific CR1000KD CR1000 Measurement and Control System - Page 325

Section 8. Operation Figure 104: Current limiting resistor in a rain gage circuit 8.1.10.3 RS-232 Sensors RS-232 sensor cable lengths should be limited to 50 feet. 8.1.10.4 SDI-12 Sensors The SDI-12 standard allows cable lengths of up to 200 feet. Campbell Scientific does not recommend SDI-12 sensor lead lengths greater than 200 feet; however, longer lead lengths can sometimes be accommodated by increasing the wire gage or powering the sensor with a second 12-Vdc power supply placed near the sensor. 8.1.11 Synchronizing Measurements Timing of a measurement is usually controlled relative to the CR1000 clock. When sensors in a sensor network are measured by a single CR1000, measurement times are synchronized, often within a few milliseconds, depending on sensor number and measurement type. Large numbers of sensors, cable length restrictions, or long distances between measurement sites may require use of multiple CR1000s. Techniques outlined below enable network administrators to synchronize CR1000 clocks and measurements in a CR1000 network. Care should be taken when a clock-change operation is planned. Any time the CR1000 clock is changed, the deviation of the new time from the old time may be sufficient to cause a skipped record in data tables. Any command used to synchronize clocks should be executed after any CallTable() instructions and timed so as to execute well clear of data output intervals. Techniques to synchronize measurements across a network include: 1. LoggerNet (p. 77) - when reliable telecommunications are common to all CR1000s in a network, the LoggerNet automated clock check provides a simple time synchronization function. Accuracy is limited by the system clock on the PC running the LoggerNet server. Precision is limited by network transmission latencies. LoggerNet compensates for latencies in many telecommunications systems and can achieve synchronies of