The Next Three Things To Immediately Do About RS485 Standard > 자유게시판

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To be on the conservative side, it is always best to use dual twisted pair shielded transmission cable for RS485. However the advantages of RS422, RS485 standard and even more importantly RS485 make their application important to the C/MRI. You need to upgrade to RS485 or RS422, and later I will show you how to build an RS232 to RS485 converter card. The 4-wire design, as implemented by the C/MRI, takes advantage of both worlds, RS422 and RS485, by dedicating one set of the more advanced RS485 transceivers to sending data from the PC and a second set for handling data being received by the PC. 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. Because typical computer interfacing distances are very short compared to the 4000-foot limit of RS485, the type of cable used is not overly important.

Section 1.3, Data Signalling Rates, states; this standard is applicable for use up to a nominal limit of 20,000 bits per second. Section 4 defines the electrical characteristics of the generator (transmitter or driver), receiver, transceiver, and system. RS232 is limited to one receiver and one transmitter. When it was able to receive more data, the receiver would use the handshaking lines to signal the transmitter that it was OK to send more data. Another RS-232 to RS485 scheme is to monitor the data stream going into the RS-485 transmitter and trigger a one-shot timer when an edge occurs. Additionally, because RS485 was developed later than RS422, the RS485 transceiver ICs, a combination transmitter and receiver, exhibit higher input impedance and output drive capability. The principal exception is that the RS485 specification is Half-duplex, meaning that it cannot send and receive data simultaneously. In a computer system, SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive. Also, for applications where you desire to filter out high frequency noise, it is better to use tantalum, disk ceramic or monolithic capacitors in place of the frequently lower-priced electrolytic capacitors, which are less effective at the higher frequencies.

Devices may be designed to operate at lower data rates for "economically specific applications". In a serial interface the receiving and transmitting devices must operate at the same baud rate. If they don't communicate, reducing the bit rate may make the two devices function together. The 4-segment DIP switch, SW2, sets the baud rate with all segments off corresponding to 9600 bps. This is achieved by giving each SMINI a unique address, 0 through 127, set by using the 7-segment address DIP switch SW1. Each switch is separately controllable by software giving you total freedom to turn each circuit on and off as desired. Each additional node is simply daisy-chained, with the same 4-wire cable running from the computer to the first node, on to the second, to the third and so on up to a possible total of 16 nodes. Even if you use 1% resistors, the standard values would be 127Ω and 1.47kΩ. This would result in a total termination resistance of 121.7Ω and a bias voltage of 0.207V. Since the tolerance of the resistors means that you will never get the ideal calculated values, don't sweat it. For example; at a higher frequency the model's series resistance could increase from 0.5 Ω to 1 Ω (per wire) and the parallel resistance could decrease from 14.52k Ω to 7.32k Ω.

This is significant because a serial interface transmits data sequentially, one bit after another, as opposed to a parallel interface, which for example might transmit one byte (eight bits) at a time. Because there can be up to 128 SMINI cards connected on the same 4-wire cable, the system must be able to discern between which SMINI is involved with communications at any point in time. And, because we are using the more advanced RS485 ICs, we expand our maximum node capability from 16 to 128. Because of this doubling up of the number of RS485 transceiver ICs and the corresponding use of 4-wires, this arrangement is typically referred to throughout the commercial and industrial world as a "Full Duplex" implementation of RS485. With the exception of some versions of the Macintosh that provide a direct RS422/485 connection, few if any other personal computers directly support RS422 or RS485. Whereas RS232 can support communication between only two devices, an RS422 port can be connected with up to 16 devices, or nodes. However, just because the 4-wire arrangement has the capacity for supporting simultaneous communication in both directions, it does not mean that the capability is always utilized, as is the situation with the C/MRI.