The CAT-5 cable allows you to relatively quickly switch -- to half-duplex RS-485, or the 3 wires of RS-232, or a variety of other communication protocols -- without pulling any new cables. But a lot of RS-485 hardware uses only 1 pair of wires (half-duplex). I've been told that 10BASE-T Ethernet and SCSI cables use a bunch of RS-485 pairs -- is that right ? This is why RS-485 states "The use of a cable termination is normally required".. 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. 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. Some RS-232 to RS-485 converters originally used the handshaking signals such as RTS to control the enable of the RS-485 transmitter. Is there a similar standard for RS-485 on a RJ45? If the data bit is sampled while there is ringing it may be sampled as an incorrect value.

A 120 Ω resistor on a 100 Ω cable will dramatically reduce the ringing compared to no termination. A termination resistor can increase the signal's quality (reduce ringing), but it will also reduce the signal's amplitude. Biasing will also improve noise rejection. This provides some noise immunity as well as resolves the problem of missing the start bit, but only for the receivers that implement this internal biasing. 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. A single set of bias resistors could be used at one end of the network, if the resistance was halved to 720 Ω. In the case of 8 bit SCSI this is not the case, the drivers are single wire with a 220/330 Ω terminator at each end of the buss. 5V is shown in the figure since many drivers are powered by 5Vdc. Other pull-up voltages can be used and it is not an absolute requirement that the pull-up voltage match the driver's supply voltage.

As mentioned above in the Voltages section; On the RS-485 network there will be times when the two wires are not driven by a transmitter. Fail-safe is mentioned twice in RS-485. Don't assume that the A and B pins of an IC match the "A" and "B" of the RS-485 standard because they can't. TODO: add connectors and pins from manufacturers such as NI, B&B, ADAM, etc. Volunteers? Many other IC manufacturers make a drop-in replacement for this IC, and many of them (such as the MAX483) use the same labeling. Normally you will want to use a pull-up voltage that matches the driver's supply voltage. There is no way to say that a termination resistor will always increase the line length the network. A final thing to note is that this model falls apart if the lines are not of infinite length. If both the driver and receiver of the devices on the network have no inversion, (or If both the driver and receiver of the devices are inverted) then the A and B lines of the devices should be connected together.

The stop sending / OK to send lines would toggle on and off with every byte sent. And since a 120 Ω termination resistor can cause enough signal loss with an extremely long 120 Ω cable to stop the network from functioning, a 500 Ω or even a 1kΩ termination resistor may improve the signal quality enough without causing too much signal loss for the network to function. This is too complicated a subject to go into depth here but a basic tutorial using oversimplified concepts may help. Termination is a less controversial subject than a misunderstood subject. If you are pushing any limits, the termination resistor value has to be determined empirically for the individual network. These examples are for bias resistors that just barely meet the 0.2V differential voltage. If you can get within 10%, you can expect it to work. You can design the system to accomplish this if you pay careful attention to logic function of the driver being used, as well as the labeling of the terminal on the device. The designer has to define the logic function of the driver and receiver. 12 V relative to the signal ground.

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