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The higher the data rate and the longer the cable, the more likely it will be that you have to add termination resistors. Lower data rates have more time for the ringing to stabilize so the lower the data rate the less needed this resistor is. Since data rates on a PC serial port can vary from 300 baud (26mS for 8-bits) up to over 100k baud (0.08mS for 8-bits), the timer will have to either be limited in the baud rates that it can work with, or keep control of the RS-485 network for much longer than it needs. From a hardware point of view, full-duplex RS-485 has some major advantages over RS-232 -- it can communicate over much longer distances at higher speeds. A long 3-conductor cable intended for RS-232 can often be switched to half-duplex RS-485, allowing communication at higher speeds and at higher external noise levels than the same cable used with RS-232 signaling. At the other end of the cable is (typically) the same thing -- the connectors, the RS-485 interface IC, and a UART inside a microcontroller.

This applies to the 8 bit version of the original 50 way SCSI interface. There is no way to say that a termination resistor will always increase the line length the network. But this is only true if there is no ringing or noise on the line that switches the receiver back to a 0 at the end of the stop bit. The diagram below shows potentials of the A (blue) and B (red) pins of an RS-485 line during transmission of one byte (0xD3, least significant bit first) of data using an asynchronous start-stop method. This scheme will automatically take control of the network when a byte is transmitted, RS485 standard but there will not be any guaranteed marking time so bias resistors may be required. These examples are for bias resistors that just barely meet the 0.2V differential voltage. Which fault conditions are to be detected and what is to be done when a fault is detected is application-dependent. Determining which fault has occurred would take additional hardware. The receiver can monitor the wires and indicate a fault if a 0 is detected on the wires for an extended period of time.

The first time is in the total load limit stating that the total load limit of the network including fail-safe provisions, should be no less than 54Ω. This implies that the fail-safe provisions are resistance external to the receiver. These examples are for a single 120 Ω termination. The bias resistance needs to be taken into account when selecting the termination resistor. This would ensure that the network has a period of marking before the start bit was transmitted, eliminating the need for a bias resistor to force the network to a marking state. However, an incorrect value termination resistor will usually improve the signal quality compared to an unterminated network. However, if the receiver considers the undriven wires to be a binary 0, when the driver is turned on and set to transmit a start bit, which is also a binary 0, the receiver will not see a transition, and therefore will not see the start bit. Forcing the wires to the idle condition when no driver is active is the most common reason to need biasing on the network. The software would set the RTS pin from the serial port active before it would stuff a byte into the UART's transmit buffer.

Look at the data sheets from each manufacturer to determine what signal is on what pin and if the two devices can be connected either directly or with an adapter. Two devices from different manufacturers that adhere to the EIA-449 standard should be able to be directly connected together. If you've been paying attention, you may notice that the last two points are contradictory. PC environment. 8P8C modular connectors are used in this case. Many connectors and pin assignments exist, but there is no standard. The connections range from screw terminals that bare wire is inserted into, to connectors such as a DE-9. It also had over 10 miles of wire (WOW). Biasing, sometimes called fail-safe biasing, is discussed briefly in the RS-422 and RS-485 specifications. ↑ "LTC1685 - 52Mbps, Precision Delay, RS485 Fail-Safe Transceiver". ↑ "Guidelines for Proper Wiring of an RS-485 (TIA/EIA-485-A) Network". When wiring a RS-485 network, always connect "A" to "A", "B" to "B", and "G" to "G". If this is required, it can be accomplished by any means desired including RS-485 drivers on an additional network, RS-232 drivers on extra wires, TTL levels, or really anything you can dream up since it won't work with any other RS-485 network except the one you are designing.