Super Mini-Node Interface Card (SMINI) - Part 1 > 자유게시판

If you desire more I/O than provided by a single SMINI, you can simply distribute additional SMINI cards around your layout to build up to whatever level of capability you desire. RS422 receivers. One diode between the I/O and Vcc, and another between the I/O and ground. The number of loads, the termination resistance 60 ohms (120 ohms per side), and the required (turn-on) current of the receivers. 60 ohms (120 ohms per side), and the required (turn-on) current of the receivers. The required load is 5volts / 3.5mA, which results in a load impedance of (around) 1428 ohms. A number of devices are being produced which represent 1/4 or 1/8 the unit load. If you are going to connect a wireless unit directly to a PC or if you plan on future interactive displays you should choose the USB485B. The USB485B has a built in voltage booster that provides power for LOR accessories such as the RF-V4 wireless unit. This unit is ideal for monitoring and checking the signals on a standard RS422 and RS485 data bus. Each of the USB485i devices come with Windows and Mac OS compatible drivers and are designed to meet stringent performance requirements and be easy to use while maintaining compatibility with standard serial communications boards.

Characters may be lost if the transmit driver is turned off while pending characters are still being transmitted. Similarly, when establishing the transmit mode, the application software is responsible for ensuring that the RS485 receiver is not disabled until all expected characters have been received. Done that establishes the RS485 receive mode is coded such that it waits until all queued (pending) characters have been transmitted before the driver chip is taken out of transmit mode. An EIA-422 driver can support up to 10 loads, at 4K W per load. Single board solutions may be hooked up as shown, but chassis to chassis systems require a common ground connection to run between the driver and receiver. The common name for this interface is RS422. Cabling is not specified in RS422 or RS485, but a rule of thumb is listed above for 24 AWG twisted pair copper wire with 16pF/ft. Refer to 24AWG Attenuation vs Frequency Chart.

You also avoid the need of an oscilloscope to check for the direction of data flow and the possibility of device failure causing bus contention whereby data tries to flow in both directions simultaneously on the same pair of wires. In the Widgetlords Electronics product lines that include a RS485 driver, the direction whether Receive or Transmit is controlled directly via a GPIO line. Pull-up/Pull-down resistors (Idle-line failsafe) at one end of the 485 bus may be used to bring the line voltage to a steady state (200mV) value at the end of a transmission; when all drivers are in the passive state. Transmit and Receive LED's are also provided to indicate state of traffic on the bus. Full opto-isolation is provided to protect your computer and also the data bus. The purpose of handshaking is for the receiver to tell the transmitting device to "shut up, I'm full of data and will lose anything more you send me". Last, but not least, is a large bulk capacitor (C203) to give the board a little more breathing room when dealing with sudden current draws. Looking at RC5, Pin 24, the Transmit Enable (TXEN) line, this is a little more complicated.

EIA-485 specifies bidirectional, half-duplex data transmission. Although there is significant added complexity in controlling Half-duplex data flow with RS485, its advantage is that the 4-wire requirement with RS422 is reduced to 2-wires with RS485. An RS232 interface is a system based on voltage levels and performs optimally in situations where there are minimal differences in ground potential. There are different serial communication standards such as RS232, RS422 and RS485. Additionally, in every case, the RS422 or the RS485 full duplex implementation is used to make the "daisy-chain" connection between multiple nodes. In its simplest form, a pair of converters from RS232 to RS422 (and back again) can be used to form an "RS232 extension cord." Data rates of up to 100K bits / second and distances up to 4000 Ft. RS-485 usually has a single pair of wires that both the transmitter and receiver are connected to. Receiver(s) inputs at a minimum of 200mV differential. Many (on-line) App-notes use 12K ohms as an input impedance, 60 ohms (as a termination), 3.5 mA as the minimum 'load' current, and 10 loads.

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