How Fast can you Go?

The difference in earth ground potentials between these two computers may be tens or even hundreds of volts. If an RS-232 to RS-485 converter is used, things start to get even more murky. Even though the logic function of the generator and receiver are not defined by RS-485, it makes sense to many engineers to have a binary 1 appear on the RS-485 wires when a binary 1 is being transmitted. 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. If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans. 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).

RS-422 voltages are referenced differentially from one wire to the other, but are also referenced to circuit common. RS-232 typically has a transmit wire, receive wire and signal common wire. RS-232 uses point-to-point unidirectional signal wires: There are only two devices connected to a RS-232 cable. 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". Other standards may define the connectors and signals on the pins of the connector. This avoids conflict with both Ethernet (Pins 1-3, 6) and analog phone (Pins 4-5). Grounding can be problematic depending on the application. The receiver is designed to operate with voltages between 3 and 15V in magnitude (i.e. both positive and negative voltages) but must be able to handle an input of 25V without damage. For an on, space or logic 0 state, the driver's A terminal is positive relative to the B terminal. The standard does not define the logic function of the driver or receiver. For each device interface, the driver and receiver are both connected to these two wires. If A is negative with respect to B, the state is binary 1. The reversed polarity (A positive with respect to B) is binary 0. The standard does not assign any logic function to the two states.

Yet the signaling waveforms in Figure 1 of the RS-485 standard clearly show that when a binary 0 (ON) is on the wires, the voltage on wire "A" is positive with respect to "B" and conversely, when a binary 1 (OFF) is on the wires, the voltage on wire "A" is negative with respect to "B". Figure 1 also shows the output signaling waveforms, redrawn here in color for clarity. Conversely, RS485 standard the output that goes low when the input goes high is often called the "inverting" output. The opposite is true of an inverting output. Logic IC's don't output a precision voltage. For normal logic when the input of a driver is a 1 or high, the non-inverting output will go high. As a general rule when the voltage measured (with respect to circuit common) is "high" the binary state is considered to be a 1 and when the voltage is low the state is 0. There are exceptions to this rule (such as differential logic and negative logic), but for the purpose of this discussion we'll talk about normal logic. 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 logic function of the driver and receiver are not defined, only the binary state of the differential voltages on the wires. When it was able to receive more data, the receiver would use the handshaking lines to signal the transmitter that it was OK to send more data. Each pair has two wires to which a driver and one more receivers are connected. When the driver increases the voltage on one of the wires, it simultaneously decreases the voltage on the other wire resulting in a differential voltage between the two wires. Two Tibbo Project System (TPS), Gen. 2-compatible Tibbo BASIC applications for the setup and testing of Bus Probes are available: BP-Tester-UI and BP-Tester-Web. This is a good first step if the communications are not working. The TX output of a first device connected to the RX input of a second device, and the TX output of the second device connected to the RX input of the first device.