アプリケーションノート 2116

絶縁フルデュープレックス動作のRS-485データインタフェース


要約: 2つの集積回路(IC)のみで完全絶縁型RS-485フルデュプレックスデータ通信を提供する回路について記載されています。データ経路の内部トランス結合絶縁型電源と内部フォトカプラは1分間につき1600Vrmsまでの、または1秒間につき2000Vramの標準絶縁を提供します。ICの選択により、250kbaudのデータレートは内部スルーレート制限回路で達成され、2.5Mbaudまでのデータレートは、非スルーレート制限回路で達成されます。

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.
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.

次のステップ
EE-Mail EE-Mail配信の登録申し込みをして、興味のある分野の最新ドキュメントに関する自動通知を受け取る。
Download ダウンロード、PDFフォーマット
© , Maxim Integrated Products, Inc.
このウェブサイトのコンテンツは米国および各国の著作権法によって保護されています。コンテンツの複製を希望される場合は お問い合わせください。.
APP 2116:
アプリケーションノート 2116,AN2116, AN 2116, APP2116, Appnote2116, Appnote 2116