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The driver has the capability of driving 10 receivers of 4k impedance, but the actual number that can be driven depend on the actual input impedance of the receivers, bit rate, wire, stub lengths, biasing and termination of the network. In a RS-232 cable, data always flows in only one direction on any particular wire, from TX to RX. The driver must output between 5V and 15V in magnitude into a load of 3kΩ to 7kΩ. The driver must not be able to output more than 100mA when shorted to any other conductor in the cable, must not be able to output more than 25V and must be able to handle an open circuit, or a short to any other conductor in the cable. Now look at the driver schematic symbol you should notice that the "A" output is the non-inverting output and the "B" output is inverting. A binary 1 may (or may not) be inverted by the driver before it is output.

This will cause reflections both back to the driver and forward to other receivers on the cable. It then travels back to the source and will reflect off of it (at the driver's end). The entire subject of handshaking is obsolete with modern USB to RS-485 adapters since the PC's driver's and/or hardware handle driver enabling, and a modern PC can handle data rates much faster than can be transmitted over an RS-485 network. 4 (DI - the driver's input pin) is high, the output voltage on pin 6 (The A output) will be positive with respect to pin 7 (the B output). The inverting output usually is shown with a bubble on it and the non-inverting output does not have the bubble. Conversely, the output that goes low when the input goes high is often called the "inverting" output. When the input is high the inverting output goes low, and when the input is low the inverting output goes high. The truth tables of most popular devices, starting with the SN75176, show the output signals inverted. The "pin mislabeling" discussed above is a good starting point. This is a good first step if the communications are not working.

A final thing to note is that this model falls apart if the lines are not of infinite length. There is no way to say that a termination resistor will always increase the line length the network. Well if you paid attention to the previous discussion, there is no way. They can be send from master/Raspberry/BananaPi to the laptop, but not the other way around. The LMOD or L485 boards can be configured for full-duplex or half-duplex operation using jumpers. Systems of this type (4-wire, half-duplex) are often constructed to avoid "data collision" (bus contention) problems on a multi-drop network (more about solving this problem on a two-wire network in a moment). There are many cables available meeting the recommendations of RS-422 and RS-485, made specifically for that application. In this implementation the on the wire format has been modified to use the Lego defined baud rate of 921600 bps and uses byte stuffing rather than bit stuffing (since there is no hardware support).

The stop sending / OK to send lines would toggle on and off with every byte sent. 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. Stating that the logic function of the generator and receiver are not defined, then showing a symbol and signaling waveform of the wires that are inverted, adds more confusion. The inductance, resistance, and capacitance of the wires are modeled below. When 5V drivers are used, the driver typically pulls one wire to common, and the other wire to 5V (and vice-versa for the opposite data). 10V difference. Beware of ground loops when using the shield as the third wire. The receiver's input impedance is specified in terms of a "unit load" where a unit load is specified as input current in mA at a voltage referenced to ground. The desktop is connected to earth ground and the RS-485 port is referenced to the earth ground.

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