New-Tech Europe Magazine | June 2019

Standard

RS-232

RS-422 Differential

RS-485 Differential

Line configuration

Single ended

Half duplex (2 wire) Full duplex (4 wire)

Type of transfer

Full duplex

Full duplex

TX, RX, RTS, CTS, DTR, DSR, DCD, Ground

TXA, TXB, RXA, RXB, Ground Data A, Data B, Ground

Signals used

Bus topology

Point-to-point

Point-to-point

Multi-point

Maximum connected devices 1

10 (in receive mode)

32

Maximum length

15 meters at 9600 bits/s

1.2 km at 100 Kbits/s

1.2 km at 100 Kbits/s

Maximum data rate Receiver sensitivity

1 Mbit/s ±3 volts

10 Mbits/s

10 Mbits/s

±200 millivolts ±200 millivolts Table 2 : A comparison of the characteristics of the RS-232, RS-422, and RS-485 standards. (Table source: Digi-Key Electronics)

such single-ended connections, signal voltages are measured from the line to ground. In the industrial environment there is a lot of noise that gets picked up by the RS-232 signal lines, which limits the length of the bus runs. A classic way around this limitation is to use differential signaling. A differential bus consists of two wires for each signal, where signals are measured by taking the voltage difference between the two wires.

Since noise and crosstalk are generally common to both signal lines, the difference measurement subtracts these nearly identical signals, significantly reducing their amplitude. Additionally, differential cables are also shielded to further reduce noise and interference pickup. There are two common data bus standards that use differential signal lines: RS-422 (TIA/EIA-422) and RS-485 (TIA/EIA-485), with the

latter being the most commonly encountered industrial serial bus. These standards use twisted pair transmission lines where connected devices can be up to 1200 m (4000 feet) apart. Both standards have maximum data rates of up to 10 Mbits/s. A comparison of all three serial buses is shown (Table 2). RS-422 and RS-485 differ in that RS-485 can work with up to 32 transceivers (more can be added using bus extenders) while RS-422

Figure 6: The full duplex (left) and half duplex topologies of an RS-485 interface. The computer or master device is shown in red; other devices are in blue. (Image source: Texas Instruments)

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