Any receiver connected at some point in the cable will change the impedance of the cable at that point on the cable. EIA-485 requires a 120 Ohm line impedance (normally Shielded Twisted pair 'STP'), 120W (10%, 1/2 watt) terminations at both ends of the line (at the Receivers). The required load is 5 volts / 3.5mA, which results in a load impedance of (around) 1428 ohms. An ESD shock to the laptop could cause the two RS-485 wires to increase in potential against the earth ground to several thousand volts. Pull a cable between the two devices and see if they can talk to each other. The CB485 can be used with a wide number of RS-485 converter ICs using the common MAX483 and MAX485 half-duplex 8-pin DIP pinout. A number of specifications use EIA-485 as the electrical standard, including SCSI-2, SCSI-3, BitBus, and Profibus. It is very handy for anything that has an Ethernet port for the ModbusTCP, including devices like Raspberry Pi based controllers industrial controllers. Up to 32 transmitters and 32 receivers may be interconnected in any combination, including one driver and multiple receivers (multi-drop), or one receiver and multiple drivers.
Repeaters may be used to extend the net to any number of devices. This protocol may include things like the start bit, number of data bits, parity and stop bit(s). Pull-up/Pull-down resistors (Idle-line failsafe) at one end of the 485 bus may be used to bring the line voltage to a steady state (200mV) value at the end of a transmission; when all drivers are in the passive state. Receiver(s) inputs at a minimum of 200mV differential. Balanced (differential) interface; defines the Physical layer, signaling protocol is not defined. But some device manufacturers match their A and B labels to the symbol (no inversion), and some match to the signaling waveform (inverted). This would enable the same standards to be used worldwide and also give manufacturers access to a global market using just one product. Now being used commonly in the pro audio industry to control digital audio and signal processors such as the DBX driverack and other manufacturers equivalent products. In summary, the EIA/TIA standards (which are commonly miscalled RS standards) are the physical layer which include electrical signal levels and RS-232 includes the connector. If the signal is unstable, consider using a bias resistor at a selected point in the line.
But this is only true if there is no ringing or noise on the line that switches the receiver back to a 0 at the end of the stop bit. EIA-485 devices may be used in 422 circuits, but EIA-422 may not be used in 485 circuits (because of the lack of an Enable line). However the other standards that have been introduced more recently provide higher levels of performance that are very useful in may applications. Single board solutions may be hooked up as shown, but chassis to chassis systems require a common ground connection to run between the driver and receiver. Devices utilizing RS485 can communicate with central control systems using communication protocols such as Modbus and ASCII. I have discovered a nice CLI tool called mbpoll than can be used for fast Modbus wiring validation via CLI. Many engineers have worked on an RS-485 application and think that since their application worked, they understand exactly what the standard is. While a multi-drop "type" application has many desirable advantages, RS422 devices cannot be used to construct a truly multi-point network. The common name for this interface is RS422. In addition to this, RS485 extends the common mode range for both drivers and receivers in the "tri-state" mode and with power off.
Normally required once the system has reached it's quiescent state, when no drivers are driving the bus. The resistors are normally required once the system has reached it's quiescent state, when no drivers are driving the bus. The current sense resistors must have short, relatively straight traces to their respective pins. Yet few of these engineers have ever seen the standard, let alone read it. Read more about RS232. Many RS422 devices are compatible with RS232. Read more about RS422. The purpose of handshaking is for the receiver to tell the transmitting device to "shut up, I'm full of data and will lose anything more you send me". This was developed in 1962 when the need for forms of transmitting data from modems attached telephone lines to remote communications equipments became apparent. The RS-232, RS-422, and RS-485 standards are not comprehensive communications protocols, but are electrical layer standards that are intended to be used in conjunction with other standards. With RS232 well established and the need for faster communications and over longer distances, further standards beyond RS232 were introduced.
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