If you want to connect a rugged tablet or laptop to measuring instruments or drive controllers, the device datasheet often lists only a serial port to RS-232. The equipment on the other end, however, calls for RS-485 or RS-422. The three names stand for electrical standards, not for connectors or protocols. This article compares RS-485 and RS-422, gives range, cabling and termination, and shows which devices in the RobustBook range have such an interface according to their datasheets. There are four, all from RuggON. Every other device needs an adapter. The standard values come from Texas Instruments application reports, the device values from the datasheets.
RS-485 and RS-422 are officially called TIA/EIA-485 and TIA/EIA-422. Both standards describe only the electrical characteristics of drivers and receivers. The signal travels as a voltage difference over two lines, not against a common ground. Connectors and pin assignment are not covered, and neither is a protocol. As an example of a protocol above them, Texas Instruments names the Chinese standard DL/T645 for energy meters, which specifies RS-485 as the physical layer.
The difference lies in the number of participants. Under RS-422, one device sends to up to ten receivers. RS-485 allows several drivers on the same pair, but only one at a time, and up to 32 unit loads on the bus. One unit load corresponds to a load of about 12 kΩ. Frugal chips take only an eighth of that, so up to 256 participants can be connected. RS-485 chips also work in RS-422 networks, but not the other way round.
The fact that the difference counts has a practical benefit. On a twisted pair, external interference couples equally into both lines. The receiver forms the difference and suppresses that share. Both standards require the receiver to switch reliably from ±200 mV of differential voltage. In common-mode range RS-485 is more generous: it reaches from -7 V to +12 V, while RS-422 has ±7 V.
The table sets the three standards side by side using the values from the Texas Instruments reports. RS-232 appears only as a comparison column. Connection and troubleshooting for RS-232 are covered in the article RS-232 in practice: connection, pin assignment, troubleshooting.
| Feature | RS-422 | RS-485 | RS-232 (comparison) |
|---|---|---|---|
| Signalling | Voltage difference on two lines | Voltage difference on two lines | Voltage against a common signal ground |
| Participants | 1 driver, up to 10 receivers | up to 32 unit loads, one driver at a time | 2 devices (DTE and DCE) |
| Receiver threshold | ±200 mV | ±200 mV | ±3 V |
| Maximum data rate | 10 Mbit/s | 10 Mbit/s at 40 feet (about 12 m) | 20 kbit/s |
| Cable length | not limited by the standard; usually up to about 1 km at no more than roughly 100 kbit/s | 4000 feet (about 1200 m) at 100 kbit/s | load capacitance up to 2500 pF, which is about 15 to 20 m |
| Termination resistor | once, at the end of the line by the last receiver | at both ends of the line | not part of this comparison |
The maximum values do not apply at the same time. The longer the cable, the lower the data rate must be. As a cautious rule of thumb, Texas Instruments gives that line length in metres times data rate in bit/s should stay below 10 million. At 500 m that would be 20 kbit/s. The report itself notes that modern cables manage more. For planning, this means: work with the rule of thumb and then measure.
RS-485 usually runs over a single pair. Sending and receiving then take place one after the other. After sending, the device must switch to receive mode so that another participant can answer. If two drivers switch on at the same time, the result is line contention, and the data on the bus is unusable for that period. Both standards therefore require a current limit on the drivers so that nothing is destroyed.
For simultaneous sending and receiving, RS-485 needs two pairs, that is, four wires. One pair carries the data in one direction, the second in the other. In an RS-422 network, one device sends on its pair, the receivers hang on that pair, and the return direction needs a second one.
According to Texas Instruments, industrial RS-485 cables are unshielded twisted pairs with a characteristic impedance of 120 Ω and 22 to 24 AWG. Two termination resistors of 120 Ω each suit them, one at each end of the main line. Stubs to the participants should stay as short as possible. With RS-422 the resistor sits only once, at the end by the last receiver.
Two more points lead to faults in the field. First, the idle level: when no participant is sending, the bus carries a differential voltage close to zero, and noise can make a receiver produce false characters. Resistors that pull the bus to a defined level when idle help, as do chips with built-in idle-level protection. These resistors load the bus with up to 20 unit loads and reduce the number of possible participants. Second, ground: large potential differences can exist between distant devices. Texas Instruments advises against simply joining the grounds with a wire, because equalising currents then couple into the data lines as interference, and shows galvanically isolated interfaces as the solution, which block current flow between the grounds.
A search through all datasheets in the range finds four devices that list RS-485 or RS-422. All four are RuggON tablets and give the interface in the "Interfaces" row of the core data table. In the datasheets of the other devices, the entry is either RS-232 or no serial interface, but neither RS-485 nor RS-422. The table reproduces the core data rows of the datasheets, which are based on the RuggON manufacturer datasheets.
| Device | Display, protection | RS-485 and RS-422 per datasheet | Operating temperature | Weight | System |
|---|---|---|---|---|---|
| RuggON Viking II 07A | 7 inches, IP65 | RS-422 × 1, RS-485 × 1 | -30 °C to +60 °C | 1.25 kg | Android 14 |
| RuggON Vortex 07X | 7 inches, IP65 | RS-232 (RX/TX)/422/485 × 1, RS-232 (RX/TX)/RS-485 × 2 | Atom x7211E: -30 °C to 60 °C; Core i5-1335UE: -30 °C to 55 °C | 1.67 kg | Windows 11 IoT Enterprise |
| RuggON Vulcan 10A | 10.4 inches, IP66 | RS-232 (TX/RX)/422/485 × 1, RS-485 × 2 | -30 °C to 60 °C | 3.2 kg | Android 14 |
| RuggON Vulcan 10X | 10.4 inches, IP66 | RS-232 (TX/RX)/422/485 × 1, RS-485 × 2 | -30 °C to +55 °C | 3.2 kg | Windows 10 / 11 IoT Enterprise |
The Viking II 07A is the only device with separate ports for RS-422 and RS-485, one each. It also has two RS-232 ports and weighs 1.25 kg. On the Vortex 07X, the datasheet lists one port as RS-232 (RX/TX)/422/485 and two more as RS-232 (RX/TX)/RS-485. According to the manufacturer's illustration, the first sits on the connector group IO-B (Generic), the other two on IO-C (M12). The Vortex thus has three ports with RS-485 but only one with RS-422. The Vulcan 10A and 10X also have three: one combined and two pure RS-485 ports. Both also have one RS-232 port with power supply and two further RS-232 ports.
The notation "RS-232 (TX/RX)/422/485" is the manufacturer's. How the operating mode of such a port is selected is not stated in the datasheets. They also say nothing on galvanic isolation or the termination resistor, and they give no maximum baud rate. If your installation requires one of these properties, clarify it with us and the manufacturer before ordering.
All four devices also have a CAN port, on the Vulcan 10X as an option. How CAN and SAE J1939 work alongside is explained in the article CAN bus and J1939 on a vehicle terminal. For vehicle terminals in general, read Vehicle terminal and forklift terminal.
For the Durabook laptops and tablets, the Fieldbook devices and the Getac X500, the datasheets list RS-232 but nothing else. An RS-232 socket cannot simply be attached to an RS-485 bus: one standard works with voltages against ground and a threshold of ±3 V, the other with a difference and a threshold of ±200 mV. A converter is needed between the two. Which devices have an RS-232 port is shown on the page for laptops with a serial interface, which currently exists in German only.
The usual route is a USB adapter. The Moxa UPort 1130, which offers RS-422 and RS-485, is an example of the data for such a device. In its datasheet Moxa gives baud rates from 50 bit/s to 921.6 kbit/s, the signals Data+, Data- and GND for two-wire RS-485, and the pairs Tx and Rx for four-wire operation. Only the UPort 1130I variant has 2 kV galvanic isolation. The adapter is not a RobustBook product and is only an example of the details you should look for.
An adapter narrows the device's operating range. According to Moxa, the UPort 1130 works from 0 to 55 °C, while the RuggON tablets in the table work from -30 °C. Moxa lists drivers for Windows, macOS, Linux and WinCE. Check before buying whether a driver exists for your tablet's operating system, especially on Android.
The choice depends on the equipment at the other end, not on the device. Three cases cover most installations:
The standard itself says nothing about address, baud rate, parity or telegram structure. The protocol above it sets those, and they are in the manual of the equipment at the other end. Check there first whether it expects two or four wires and whether it brings its own termination resistor. For machines in production, also read Machine control and maintenance in production and the industry page Industry.
No. Both work with voltage differences, but RS-422 allows one driver with up to ten receivers, RS-485 several drivers on the same bus. RS-485 chips can be used in RS-422 networks, whereas RS-422 chips are not reliable in RS-485 networks with several drivers.
Texas Instruments gives 4000 feet, about 1200 m, at 100 kbit/s and 10 Mbit/s at 40 feet. In between, the achievable data rate falls with length. The actual limit depends on the cable, the termination and the interference on site.
No. The signal levels and the principle differ, as the table above shows. You need a converter, either from USB or from RS-232 to RS-485.
According to the datasheets they are the RuggON Viking II 07A, the Vortex 07X, the Vulcan 10A and the Vulcan 10X. No other datasheet in the range lists RS-485 or RS-422.
If the installation calls for RS-485 or RS-422 and the rugged tablet sits in the vehicle or at the machine, the four RuggON devices are candidates. The Viking II 07A is the smallest at 7 inches and 1.25 kg and has one port each for RS-422 and RS-485. If you want to connect several RS-485 lines, you are better served by the Vortex 07X (7 inches, Windows) or the Vulcan 10A and 10X (10.4 inches, Android and Windows respectively): all three have three ports with RS-485.
Not covered is anyone who needs a Durabook or Getac laptop. There only an adapter remains, with the limits on temperature and drivers named above. What the datasheets do not say also stays open: galvanic isolation, termination resistor, baud rate. Clarify these points before you order.
Tell us the equipment at the other end, the number of wires and the protocol, and the RobustBook team will help you choose the right device.