Find out how To start out RS485 Standard

Although the C/MRI does not have a requirement for two-way simultaneous communication, there are definite advantages to its using a separate wire pair for sending and receiving data - namely reduced design complexity and easier system debug. The circuit illustrates the fundamental connection method for a two-wire wiring system. The two-wire half-duplex system permits bidirectional data transmission between two devices, but not concurrently. Whereas RS232 can support communication between only two devices, an RS422 port can be connected with up to 16 devices, or nodes. Two 5-pin headers are provided for use with RS485. 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. All of the drivers and transceivers must be able to be "made passive" or effectively disconnected from the network to allow other drivers to transmit.

This problem can also be fixed by adding bias resistors to force the line to idle condition when a driver is not connected, but this has to be done on a network basis, not a device basis. RS422 is a full-duplex system, meaning that a device using RS422 can send and receive information simultaneously. 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 a serial interface the receiving and transmitting devices must operate at the same baud rate. The SMINI and the SUSIC are designed to operate with standard baud rates of 9600, 19200, 28800, 57600 and 115200 bits per second or bps. With the ground potential problem reduced, the RS422 can send its Mark and Space signals closer together to operate reliably at higher baud rates and over greater distances. It is important to understand that all four serial interface standards allow different baud or data-transmission rates.

Thus, combining nodes using the Classic USIC with the newer SMINI and SUSIC nodes requires operating at either the 9600 or 19200 baud rates - the rates where the two ranges overlap. This range is considerably faster than the standard rates of 150, 300, 600, 1200, 2400, 4800, 9600 and 19200 provided with the original Classic USIC. Additionally, with separate send and receive wire pairs, it is easy to incorporate sending and receiving data LEDs as implemented on the JLC provided SMINI, SUSIC and RS485 cards. The fifth, or bottom pin in each RS485 header, is provided to maintain continuity for the shield normally used with RS485 cabling. 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. RS232, RS422, RS423 and RS485 are serial communication methods for computers and devices. Of these, R232 is the most popular protocol used for all major computers available today, and RS485 is one of the most versatile communication standards defined by the EIA. 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.

Also, it is generally recommended that cable be terminated with a resistor/capacitor combination equivalent to the characteristic impedance of the cable. To be on the conservative side, it is always best to use dual twisted pair shielded transmission cable for RS485. Worse is the fact that annex A states that this graph is a conservative guide based on empirical data of 24AWG telephone cable (POTS). The annex also says that certain applications may cause the resistor to fail so the installation must allow access for inspection and replacement. Lowering the bias resistor values will increase the bias voltage which will increase the noise immunity at the cost of increased current from the 5V supply. The product of the resistor and capacitor values (i.e. R x C), is called the filter time constant. Some manufacturers have avoided this confusion by changing the name of the output pins labels to X and Y. There are two things to be noted about this so called "pin mislabeling". There are also 9 separate I/O control wires - typically called port select or enable lines - joining the 877 to each of the 9 I/O buffering functions.

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