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When there is no data activity on an RS485 network (or in many instances RS422 networks, other than point-to-point), the communications lines are "floating" and, thus susceptible to external noise or interference. One transmitter and 10 receivers are allowed on a network, with a distance limit of 3600ft. RS422 was on the market prior to RS485; however, due to loading limitations, one of the best uses of RS422 is probably in point-to-point communications, such as RS232 extension cords. Even though some IC manufacturers offer light loading devices, that can accommodate 256 or even 400 nodes on one RS485 network, you may NOT want to build such a network for a few reasons. The list is automatically updated every time you plug or unplug a serial device.Each device's communications can be monitored in real time, and highly efficient and carefully optimized data processing algorithms help avoid communication delays. A three-post jumper located between the third socket and H6 configures the primary serial port for either RS232 or RS485 operation. Additionally, RS485 supports multidrop connections, meaning multiple devices can be connected to a single cable, while RS232 allows only one-to-one connections.

To provide a convenient means of attaching two grounds to the serial cable, there are several pins (labeled DGND) on the communications connector that are connected to the controller’s ground plane. It’s differential, meaning it uses two complementary signal lines to improve noise immunity. By "isolating" sections of a large network, the accumulated noise on one isolated leg is not so likely to cause a data error that will propagate to another leg of the network. With galvanic isolation the damage is generally limited to only one leg of the network, except in extreme cases of very high voltage (induced by lightening for example). It is possible to connect several RS485 circuits in parallel if the distances are below about 200 feet per leg @ 9600bps. At greater distances and higher data rates, the cable impedances add up and load the network. While handshaking is still with us, it is usually best to disable it in software (if possible) and/or "loop-back" the pairs of signals (RTS to CTS, DTR to DSR, etc.) While RS232 was rumored to be on the "way out" with the advent of many of the new communications standards, it is still alive and well today.

Maintain as much distance as possible and cross any power cable at a right angle. By converting from single-ended RS232 to differential RS422 and then, converting back from RS422 to RS232 at the other end of the line, distance and noise immunity can be greatly improved. RS485, CAN bus to build network with both of the signal/power in two wires (total bus can provide 960 watt power and 12 watt in every terminal). The RS232 protocol provides for four handshaking signals called ready to send (RTS), clear to send (CTS), data set ready (DSR), and data terminal ready (DTR) to coordinate the transfer of information. By connecting pairs of these handshaking signals together, the terminal or PC can be made to think that the QVGA Controller is always ready to send and receive data. RS422 and RS485 often use DB9 or terminal block connectors, but the choice of the connector can vary depending on the equipment. When the input resistance of RS-485 is 12Kohm, the network can connect more than 32 receivers or transmitters.

RS-485 standard conformant drivers provide a differential output of a minimum 1.5 V across a 54-Ω load, rs485 cable whereas standard conformant receivers detect a differential input down to 200 mV. Receivers on a network (RS485 or RS422) have built in hysteresis (200mV differential required to insure known state). The combination of the cable impedances and/or termination resistors will load the network and can make communications unreliable. Thus in Table 11 5 , RTS1 is connected to CTS1, and DSR1 is connected to DTR1 onboard the QVGA Controller using zero ohm shorting resistors. The QVGA Controller offers a unique addition to full duplex RS232: it can place its RS232 transmitter into a high impedance silent mode under software control. While shielding is not specified for RS485 systems, it can help in many instances. It is widely used in industrial automation, medical equipment, security systems, and many other applications. Security when and where you need it! For a more detailed discussion read WHY YOU NEED 3 WIRES FOR 2 (TWO) WIRE RS485 FOR BACNET. Later, we need to use it for Modbus Plus protocol.

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