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HCPL-4100 Datasheet(PDF) 9 Page - Agilent(Hewlett-Packard) |
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HCPL-4100 Datasheet(HTML) 9 Page - Agilent(Hewlett-Packard) |
9 / 12 page 9 Applications Data transfer between equipment which employs current loop circuits can be accomplished via one of three configurations: simplex, half duplex or full duplex communication. With these configurations, point-to- point and multidrop arrangements are possible. The appropriate configuration to use depends upon data rate, number of stations, number and length of lines, direction of data flow, protocol, current source location and voltage compliance value, etc. Simplex The simplex configuration, whether point to point or multi- drop, gives unidirectional data flow from transmitter to trans- mitter(s). This is the simplest configuration for use in long line length (two wire), moderate data rate, and low current source compliance level applications. A block diagram of simplex point to point arrangement is given in Figure 11 for the HCPL-4100 transmitter optocoupler. Major factors which limit maxi- mum data rate performance for a simplex loop are the location and compliance voltage of the loop current source as well as the total line capacitance. Application of the HCPL-4100 transmitter in a simplex loop necessitates thtat a non-isolated active receiver (containing current source) be used at the opposite end of the current loop. With long line length, large line capacitance will need to be charged to the compliance voltage level of the current source before the receiver loop current decreases to zero. This effect limits upper data rate performance. Slower data rates will occur with larger compliance voltage levels. The maximum compliance level is determined by the transmitter breakdown characteristic. In addition, adequate compliance of the current source must be available for voltage drops across station(s) during the MARK state in multidrop applications for long line lengths. In a simplex multidrop applica- tion with multiple HCPL-4100 transmitters and one non-isolated active receiver, priority of transmitters must be established. A recommended non-isolated active receiver circuit which can be used with the HCPL-4100 in point-to-point or in multidrop 20 mA current loop applications is given in Figure 12. This non- isolated active receiver current threshold must be chosen properly in order to provide adequate noise immunity as well as not to detect SPACE state current (bias current) of the HCPL-4100 transmitter. The receiver input threshold current is Vth/Rth ≈ 10 mA. A simple transistor current source provides a nominal 20 mA loop current over a VCC compliance range of 6 V dc to 27 V dc. A resistor can be used in place of the constant current source for simple applications where the wire loop distance and number of stations on the loop are fixed. A minimum transmitter output load capac- itance of 1000 pF is required between pins 3 and 4 to ensure absolute stability. Length of current loop (one direction) versus minimum required DC supply voltage, VCC, of the circuit in Figure 12 is graphically illustrated in Figure 13. Multidrop configurations will require larger VCC than Figure 13 predicts in order to account for additional station terminal voltage drops. Typical data rate performance versus distance is illustrated in Figure 14 for the combination of a non-isolated active receiver and HCPL-4100 optically coupled current loop transmitter shown in Figure 12. Curves are shown for Figure 11. Simplex Point to Point Current Loop System Configuration. |
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