So are Termination Resistors Required?
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The modem may be limited to half-duplex communications, and therefore the link between the two computers is half-duplex. For example; Two computers communicating with each other over an older modem. Other reasons that some RS-232 ports were half-duplex: some very old UARTs may be half-duplex limiting the system, and some very old computers drove the RS-232 drivers directly from the processor without a UART. This is the 7 or 8-bit protocol commonly associated with serial ports. A simple example is a query/response protocol. The appendix discusses capacitance and resistance of the cable, then gives an example calculation where the capacitance of the cable per foot (30pF/foot) multiplied by the cable's length, plus the capacitance of the receiver (100pF) gives a maximum cable length of 80 feet. Another example of misunderstanding the RS-232 standard is the 9-pin connector. This standard specifies the electrical characteristics of a single transmitter and up to ten receivers on a single pair of wires. RS-422 allows for multiple receivers on each pair, but only one driver. Each pair has two wires to which a driver and one more receivers are connected. Both the generator (driver) and the receiver of all devices on the network are connected to the two wires.
These other standards provide the protocol and other requirements that are needed for two devices to transfer information. A RS-485 interface will usually use pins 7 and 8 for the two data lines, since they comprise a twisted pair. Clearly not all of these circuits or signals are included in IBM’s implementation (very few implementations use secondary data lines or many of the other circuits). Some RS-485 implementations (in particular, some Ethernet configurations) (also some Macintosh GPIO socket) use 4 wires (2 pairs) for point-to-point communication. The other pair is dedicated to peripheral-to-PC communication. Note that the RS-422/485 standards call for "balanced interconnecting media", not twisted pair. Please note that a RS-485 network is a single pair of wires, but there is nothing in the RS-485 standard that prevents more than one RS-485 network from being used by each device. The RS-485 standard calls for a differential signaling scheme that will operate over a much longer line length than RS-232 can. Some thought should be given to changing the technology for really long line lengths.
A higher value termination resistor will reduce the DC losses associated with extreme line lengths, allowing for much longer line lengths at the cost of ringing on the wires. Adding a resistor to the end of the cable that matches the value of the cable's impedance will absorb the signal traveling down the line and reduce or prevent reflection. This means that it is necessary to wait until the signal has stabilized before the bit can be sampled. This protocol may include things like the start bit, number of data bits, parity and stop bit(s). It does not mean that this protocol is part of the standards or that only this protocol may be used with the 232/422/485 standards. The UART connected to the RS-232 driver/receiver controls the protocol of the bits being transferred. Any bit protocol such as that from a UART is specifically not included. Bit and framing protocols are in layer two of the OSI model.
Layer two of the OSI model includes the bit and framing protocol, which is outside of these EIA/TIA standards. RS-485 is the physical layer for many higher-level protocols, including Profibus and other fieldbus systems, SCSI-2, SCSI-3, and BitBus. The forward to RS-485 references TSB-89 which has topics including data signaling rate vs. The cable's length, impedance, terminations, stub lengths, and data rate will all have an impact on signal quality. This serial port was no longer a "RS-232 compatible" serial port since the connector and signal levels no longer matched the RS-232 standard. It can’t be denied that the UART IBM selected for its implementation of the serial port is considered by many as the definition of RS-232, but these parts are not in the RS-232 standard. The point-to-point full-duplex RS-485 network allows you to get the complete prototype system fully operational quickly, since it is easier to debug and more immune to certain common problems on other systems (noise problems on RS-232, turn-around problems on half-duplex RS-485, etc.). Since the driver and receiver are differential circuits, the input and output voltages are specified as differential, but these voltages are also referenced to a circuit common. 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.
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