rs485

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最近拿到1個USBtoRS485，了解相關資訊

  • 了解CH341
  • 了解RS232,RS485

更新記錄

item note
20161018 第一版

目錄

USB2.0 轉 RS485 轉換器

USB2.0 轉 RS485 轉換器

  • 工業級 USB2.0 轉 RS485 轉換器
    由CH341(USB bus convert chip)及 SN65HVD3082E(Low-Power RS-485 Transceivers)兩個組成
    CH341設定為serial mode，TEN#直接接地，致能UART TX功能 (由TNOW PIN決定: TX/RX)

    [CH341]

  • lsusb
    1a86:5523 QinHeng Electronics CH341 in serial mode, usb to serial port converter

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    [root@localhost ~]# lsusb
    Bus 004 Device 002: ID 8087:8001 Intel Corp. 
    Bus 004 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
    Bus 003 Device 002: ID 8087:8009 Intel Corp. 
    Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
    Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
    Bus 001 Device 002: ID 093a:2510 Pixart Imaging, Inc. Optical Mouse
    Bus 001 Device 003: ID 1a86:5523 QinHeng Electronics CH341 in serial mode, usb to serial port converter
    Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

USB转RS485接口

USB转RS485接口

[CH341]

TEN#直接接地所以一直允许 CH341通过串口发送 (TEN#直接接地)
TNOW用于控制 RS485 总线半双工收发器 U5 的收发状态
TNOW Hi : TE(送出UART資料)
TNOW Lo : /RE (接受UART資料)

[CH341 PIN]

其它 RS485 SCH

量測RS485

  • UART Basics
  • 量測RS485(GND接D-,探棒接D+)
    8N1,2400,送出0x33
    所以10bit約10x(1000/2400) = 4.16ms[8N1,2400]

RS485
Hi: D+ - DU ,(2V~6V)
Lo: -(D+ - D-) ,-(2V~6V)

其它相關資料

訊號通訊

  • 串列
    1個clk產生1個bit
    ex. I2C:SCL/SDA, UART:TX/RS
  • 並列
    1個clk產生1個byte(即8bit)
    需要多個腳位，不利於用於遠距離傳輸
    ex. SPI(Serial Peripheral Interface):SDI,SDO,SCK,CS
  • 在串列資料傳輸裡：
    • 單工：
      1個時間只有1用處：輸出資料(TX)，取得資料(RX)
      ex. rs485-2線
    • 雙工：
      1個時間可以同時輸出資料(TX)及取得資料(RX)
      ex. uart, rs485-4線

RS485 & RS232

規格 RS232 RS423 RS422 RS485
操作模式 單端傳輸 單端傳輸 差動傳輸 差動傳輸
線上允許之傳送/接受數目 1 Driver/1 Receivers 1 Driver/10 Receivers 1 Driver/10 Receivers 32 Driver/32 Receivers
最大傳輸距離 50 feet (15 m) 4000 feet (1.2 km) 4000 feet(1.2 km) 4000 feet (1.2 km)
最大傳輸速率 20 kb/s 100 kb/s 10 Mb/s 10 Mb/s
接收端之輸入靈敏度 ±3 V ±200 mV ±200 mV ±200 mV

TIA/EIA-232-F Electrical Specification

state interface voltage
輸出資料為1 -5V ~ -15V
輸出資料為0 +5V ~ +15V
接受資料為0 3V ~ 15V
接受資料為1 -3V ~ -15V

  • 介面使用一根信號線和一根信號返回線而構成共地的傳輸形式，
    這種共地傳輸容易產生共模干擾，所以抗雜訊干擾性弱

  • Each line should have a minimum load of 3 kΩ and a maximum load of 7 kΩ which usually is part of the receiver circuit.

  • driven voltage
    • a logic 0 is represented by a driven voltage between 5 V and 15 V
    • a logic 1of between –5 V and –15 V.
  • At the receiving end,
    • a voltage between 3 V and 15 V represents a 0
    • a voltage of between –3 V and –15 V represents a 1
  • Voltages between ±3 V are undefined and lie in the transition region (當訊號為+/-3V之間，為有問題區段)
  • This effectively gives a 2-V minimum noise margin at the receiver
[EIA-232-F]
  • RS232不能設定超過20K(即最大設定19200)
  • 232 specifies a maximum slew rate of the signal at the output of the driver to be 30 V/μs
  • This restriction, together with the fact that the drivers and receivers use a common signal ground and the
    associated noise introduced by the ground current, severely limits the maximum data throughput.
  • uart的訊息裡可有共模的雜訊(common signal)
    當傳輸快到一定程度就會吃到預留的2V空間,,此問題當線長及線的電感/電阻大時也常發生
    不過依spec的電容及長度,,最大就是20K
[Data Rate]

EIA-485

  • EIA-485（过去叫做RS-485或者RS4851）是隶属于OSI模型物理层的电气特性规定为2线，半双工，多点通信的标准
  • RS-485介面是採用平衡驅動器和差分接收器的組合，抗共模幹能力增強，即抗雜訊干擾性好
  • 介面信號電平比RS-232降低了，就不易損壞介面電路的晶片，且該電平與TTL電平相容，可方便與TTL電路連接。
  • 工业RS-485电缆分为有保护套、无保护套、双绞线、非屏蔽双绞线，符合美国22-24AWG线规的电缆特性阻抗为120Ω
  • The 485 is a balanced (differential) digital transmission line interface developed to improve on the
    TIA/EIA-232 (referred to hereafter as 232) limitations
    • High signaling rate – up to 50M bits/s
    • Longer line length – up to 1200 meters
    • Differential transmission – fewer noise emissions
    • Multiple drivers and receivers[485 Spec]

RS-485 Transceivers (SN65HVD3082E)

[SN65HVD3082ED -VP3082 ]

USB bus convert chip CH341

提供下列4種模式：

mode note
uart mode serial transfer enable, serial receive in ready etc.
printer port mod Standard USB printer port used to upgrade parallel printer, conforms to relevant USB specification.
parallel mode CH341 gives out 8-bit parallel in EPP or MEM mode.
Synchronous serial 2-wire (SCL, SDA)
4-wire (CS, SCK/CLK, MISO/SDI/DIN and MOSI/SDO/DOUT) interface

EPP(memory read/write mode)
MEM mode(memory read/write mode)

參考