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TSB-89A, Application Guidelines for TIA/EIA-485-A does not recommend using star topology. 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. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). Rs485 cables not only vary in the structure of the core but also in the number of twisted pairs. Another advantage over RS232 that I mentioned earlier is the number of connectable devices. It does not specify or recommend any communications protocol; Other standards define the protocols for communication over an RS-485 link. RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and Profibus. RS-485 and RS-422 can interoperate with certain restrictions.

Without termination resistors, signal reflections off the unterminated end of the cable can cause data corruption. Termination resistors also reduce electrical noise sensitivity due to the lower impedance. The termination also includes pull up and pull down resistors to establish fail-safe bias for each data wire for the case when the lines are not being driven by any device. Ideally, the two ends of the cable will have a termination resistor connected across the two wires. The shielding may be braided (be formed by a mesh of thin conducting wires) or be a foil (consisting of a sheet of metal wound around the conductors): the two types are equivalent. All headers are on a 0.1" grid to allow accessory proto-boards to span multiple connectors if needed. The power input connector can be a coaxial barrel connector, a 2-pin terminal block, or a MTA-100 connector or 0.1" spacing header. With Modbus, BACnet and Profibus, A/B labeling refers A as the negative green wire and B as the positive red wire, in the definition of the D-sub connector and M12 circular connector, as can be seen in Profibus guides. Many times the old DB-9 connector is used.

By using repeaters very large RS-485 networks can be formed. The greatest feature of RS485 is the fact that you can connect up to 32 devices on one system. RS232 could only handle one at a time. RS485 can handle up to 32 connected devices. Under some conditions it can be used up to data transmission speeds of 64 Mbit/s. As a rule of thumb, the speed in bit/s multiplied by the length in metres should not exceed 108. Thus a 50-meter cable should not signal faster than 2 Mbit/s. Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485. In contrast to RS-422, which has a driver circuit which cannot be switched off, RS-485 drivers use three-state logic allowing individual transmitters to be deactivated. Serial data communications standards - RS-232, RS-422, & RS-485 were widely used for data transfer providing easy connectivity for the time and they can still be found today. RS-485, like RS-422, can be made full-duplex by using four wires. Digital communications networks implementing the standard can be used effectively over long distances and in electrically noisy environments.

The cabling of the industrial communication systems (Modbus RS485) is different in some ways from the cabling used for power cabling and the electrician may experience some difficulties if he is not an expert in Modbus communication networks. Therefore for RS485 communications no crossover cable is used. For RS232a crossover cable is used where pin 1 is connected with pin 6 at the other connector, pin 2 with pin 5, etc. This interconnects the transmit and receive signals of both PLCs, and also the handshaking signals DTR and DSR. The symmetrical pin layout of the RJ11 sockets on the Unitronics PLCs allows an easy way of interconnecting the PLCs with both RS232 and RS485 communications. The diagram below shows a basic RS485 cable for use with the RJ11 socket on Unitronics PLCs. These signals are commonly called A and B in RS485 systems. The equipment located along a set of RS-485 wires are interchangeably called nodes, stations or devices. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. The normal way to implement this is with a two wire system with differential signal levels. In a computer system, SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive.