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If multiple devices start to talk at the same time, the data may be corrupted. Devices may be designed to operate at lower data rates for "economically specific applications". 12V and -7V. If the desktop's port is ESD protected damage may not occur, but there is no guarantee. The longer the wires are, the less signal there will be at the end. This allows RS-485 to transmit over longer distances than RS-232. The new IBM PC/AT serial port no longer used the RS-232 25-pin connector or signal levels, but IBM took care to ensure that the new serial port’s electrical levels would still work with existing serial port peripherals, requiring only a 9-pin to 25-pin adapter. But, since existing RS-232 compatible peripherals still functioned with it, it was still regularly called an "RS-232 port". But, RS485 standard the RS-232 connection from the computer to the modem would still be full-duplex. This computer had limited functionality with a keyboard and monitor as the primary peripherals. The modem may be limited to half-duplex communications, and therefore the link between the two computers is half-duplex. RS-485 discusses that it is used for devices up to 10Mbit/S, then says they need not be limited to 10Mbps. It also states that "the upper bound is beyond the scope of this Standard".

I.e. only one device can talk to the other devices on the RS-485 network at a time. The most significant difference is that more than one slave can talk back to the master. This means that in a master/slave configuration, a master can talk to multiple slaves, all of the slaves can talk back to the master, and every device on the network can hear every other device. A CRC or checksum can be added to the end of the data being transmitted. The UART connected to the RS-232 driver/receiver controls the protocol of the bits being transferred. RS-232 does not define a protocol, but the protocol that is almost always transmitted on these connectors is asynchronous start-stop ASCII (data from a UART). Layer two of the OSI model includes the bit and framing protocol, which is outside of these EIA/TIA standards. RS-485 does not specify any protocol, addressing scheme, or connector.

In the RS-485 standard, one wire is labeled "A" and the other is labeled "B", and the wires are twisted together (a "twisted pair"). Like much of the RS-485 standard, fail-safe operation is left wide open and implementation is not part of the standard. Again, this protocol is not part of the RS-232 standard, even though many engineers think of this protocol as "RS-232". This means that a higher-level protocol is usually used to verify that the data has been sent. When 5V drivers are used, the driver typically pulls one wire to circuit common, and the other wire to 5V (and vice-verse for the opposite data). A third point on the driver shown as "C" is labeled as a common, but is not shown connecting to any wire. This third example would be a good place to use an isolating RS-485 transceiver. This example of biasing would be a bad thing on an equivalent RS-485 network. The RS-485 standard includes the voltage levels of the two wires when a binary 0 or 1 is on the two RS-485 wires, but specifically excludes the logic function of the generator or receiver. If you keep the cable length at 70% or 80% of this limit you should expect the network to work with a true RS-232 driver and receiver.

The result of this looks like two 120 Ω resistors in parallel (60 Ω) to either end or to any point on the cable. For example; Two computers communicating with each other over an older modem. RS-485 allows for multiple devices on the two wires. RS-232 allows for both a transmit and receive circuit. In reality either or both the driver or receiver are going to meet the more modern RS-574 requirements and none of the RS-232 limits will apply. One-way resistor termination: RS-422 networks should only have one resistor at the receiver end. RS-422 is inherently simplex on each pair of wires. The other pair is dedicated to peripheral-to-PC communication. 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. Note that the RS-422/485 standards call for "balanced interconnecting media", not twisted pair. EIA-530 and EIA-449 are standards that specify a connector, pin assignments and RS-422 electrical levels.