Understanding Smart Sensors - Nomads.usp

122 Understanding Smart SensorsTable 6.1Point-to-Point and Multidrop StandardParameter EIA-232-C EIA-432-A EIA-422-A EIA-485Mode of operation Single ended Single ended Differential DifferentialNumber of drivers andreceivers allowedMaximum cable length(ft)Maximum data rate(bps)Maximum commonmodevoltageDriver output1 driver,1 receiver1 driver,10 receivers1 driver,10 receivers50 4,000 4,000 4,00020k 100k 10M 10M32 drivers,32 receivers± 25V ±6V 6V, –0.25V 12V, –7V±5V min.;±15V max.±3.6V min.;±6V max.± 2V min. ±1.5V min.Driver load ±3kΩ–7 kΩ 450Ω min. 100Ω min. 60Ω min.Driver slew rate 30V/ms max. Externally NANAcontrolledDriver output shortcircuit current limitReceiver outputresistance (on)high Z state) (off)500 mA to Vccor GNDNA150 mA to GND 150 mA to GND 150 mA to GND250 mA to –8Vor 12VNANA120 kΩ300Ω 60Ω 60Ω 120ΩReceiver input resistance 3–7 kΩ 4kΩ 4kΩ 12 kΩReceiver sensitivity ± 3V ± 200 mV ± 200 mV ± 200 mVis frequently used when sensors are connected in a system using a multiplexedbus offering data link–level multiplexing and allowing packets from differentsenders to be sent to different receivers. A higher level sensor network uses alllayers of the OSI model to provide more information and simplify the user’ssystem design and maintenance activity.6.3 Sources (Organizations) and StandardsSeveral universities have developed standards for communication. For example,the University of Michigan has proposed the Michigan parallel standard (MPS)