RS-485的数据介面可以让孤立的，全双工运作-RS-485
abstract: a circuit is described that provides fully isolated rs-485 full-duplex data communication with only two integrated circuits (ics). an internal transformer-coupled isolated supply and internal optocouplers in the data path provide typical isolation to 1600vrms for 1 minute or 2000vram for 1 second. data rates of 250kbaud are achieved with internally slew-rate-limited circuits or to 2.5mbaud with non-slew-rate limited circuits, depending upon choice of available ics.
the simple rs-485 circuit of figure 1 provides full-duplex communications (simultaneous transmission and reception) with only two essential packages (ic2 and ic3). its balanced and differential data lines are necessary for high-noise environments or for long-distance transmission between a computer and its peripherals. such transmissions are difficult, if not impossible, with the single-ended circuitry of an rs-232 transceiver.
the rs-485 standard allows for bidirectional, multi-point, party-line communications, with data rates to 10m bits/second (10mbps) and line lengths to 1200 meters. differential transmission provides noise immunity. the circuit shown features controlled-slew-rate drivers that minimize emi and the reflections caused by improperly terminated cables. it also enables error-free transmissions to 250kbps. to achieve data rates to 2.5mbps, substitute a full-slew-rate max1480a for ic2, a max485 for ic3, and r2—r5 values per table 1.
figure 1. ic2 and ic3 provide full-duplex data communications for cable lengths as long as 1200 meters.
ic2 is a complete half-duplex interface that includes transceivers, optocouplers, a power driver, and a transformer. the optocouplers transmit digital signals across the internal isolation barrier, and the center-tapped transformer transmits power across the barrier from its logic (non-isolated) side to its isolated side.
ic3, powered by the isolated vcc, upgrades the half-duplex operation of ic2 to full duplex using ic2's own dedicated optocouplers. pin 3 must be tied low to disable ic3's driver, and pin 4 should be left floating. the driver outputs for ic2 and ic3 exhibit high impedance when active-low de is low; bringing active-low de high enables the outputs to function as line drivers.
the isolation barrier in ic2 typically withstands 1600vrms for one minute or 2000vrms for one second. any ttl/cmos-logic family can drive the ic2 digital inputs through a series resistor. with resistive pull-ups, the receiver outputs can drive any such logic as well. ic2's isolated outputs meet all rs-485 specifications.
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the simple rs-485 circuit of figure 1 provides full-duplex communications (simultaneous transmission and reception) with only two essential packages (ic2 and ic3). its balanced and differential data lines are necessary for high-noise environments or for long-distance transmission between a computer and its peripherals. such transmissions are difficult, if not impossible, with the single-ended circuitry of an rs-232 transceiver.
the rs-485 standard allows for bidirectional, multi-point, party-line communications, with data rates to 10m bits/second (10mbps) and line lengths to 1200 meters. differential transmission provides noise immunity. the circuit shown features controlled-slew-rate drivers that minimize emi and the reflections caused by improperly terminated cables. it also enables error-free transmissions to 250kbps. to achieve data rates to 2.5mbps, substitute a full-slew-rate max1480a for ic2, a max485 for ic3, and r2—r5 values per table 1.
figure 1. ic2 and ic3 provide full-duplex data communications for cable lengths as long as 1200 meters.
ic2 is a complete half-duplex interface that includes transceivers, optocouplers, a power driver, and a transformer. the optocouplers transmit digital signals across the internal isolation barrier, and the center-tapped transformer transmits power across the barrier from its logic (non-isolated) side to its isolated side.
ic3, powered by the isolated vcc, upgrades the half-duplex operation of ic2 to full duplex using ic2's own dedicated optocouplers. pin 3 must be tied low to disable ic3's driver, and pin 4 should be left floating. the driver outputs for ic2 and ic3 exhibit high impedance when active-low de is low; bringing active-low de high enables the outputs to function as line drivers.
the isolation barrier in ic2 typically withstands 1600vrms for one minute or 2000vrms for one second. any ttl/cmos-logic family can drive the ic2 digital inputs through a series resistor. with resistive pull-ups, the receiver outputs can drive any such logic as well. ic2's isolated outputs meet all rs-485 specifications.
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