RS485 cable type and speed

The maximum permissible bus length depends on the transmission rate and the bus cable type. Different cable types (designation A to D) for different applications are available for connecting PROFIBUS devices either each other or to network elements (Linking Devices, Repeaters).

The electrical lines are shielded twisted two-wire cables with a circular cross-section. The RS 485 interface operates on voltage differences. It is therefore less sensitive to interference than a voltage or current interface. Depending on the transmission speed, segments can be implemented in lengths of from 100 meter to over 1000 meter. The bus cable to be used for the bus modules is specified in DIN 19245/EN 50170 as cable type A and can be used according to the following table:

Parameter	DP, Cable Type A
Cable Design	Twisted pair and shielded
Surge Impedance	135 … 165 Ohm at f =3…20Mhz
Operating Capacity	=<>
Loop Resistance	=<>
Wire Diameter	>0.64 mm
Wire Cross-section	>0.34mm2

From the line parameters specified above result the following bus segment lengths:

Transmission Rate in kbits/sec	9.6	19.2	93.75	187.5	500	1500	12000
Range in Meter/Segment	1200	1200	1200	1000	400	200	100

NOTE: In a PROFIBUS DP installation, a data transfer rate must be chosen which is supported by all devices connected to the bus. The chosen data transfer rate then determines the maximum segment lengths as shown above.

If other transmission rate shall be used which is not listed in the table above, an approximation can determine the permissible length through linear interpolation of two adjacent transmission rates from the table.

Calculation of a bus length against of two transmission rates:

A transmission rate of 3 Mbit/s shall be used. The transmission rates in the table close to this value are 1.5 Mbit/s with l 1.5 Mbit/s and 12 Mbit/s with l 12Mbit/s. Thus, you can calculate the line length l 3Mbit/s:

RS485 network limits

Up to 32 nodes (master and slaves) can be grouped in a single segment. When using more than 32 nodes, several segments linked to each other through repeaters (power amplifiers) are needed. Note that the repeater has to be considered as another bus node, since the integrated bus drivers are an additional load on the bus. It is recommended to use not more than three series-connected repeaters on a line. However, the number of repeaters in a single PROFIBUS network is limited to 9.

The maximum admissible distance between two bus stations in each PROFIBUS network can be calculated as follows:

(Number of repeater + 1) * Segment length

Number of repeater = The maximum number of repeaters connected in series.

Example:

The repeater manufacturer’s specifications allow nine repeaters to be connected in series.

The maximum distance between two bus stations at a data transfer rate of 1500 kbit/s is then as follows: => (9 + 1) * 200 m = 2000 m

Adding field devices to the RS485 network

A shielded twisted-pair copper cable is the hardware required for RS485 transmission. It connects all devices to the bus structure (line). The bus structure allows for non-reactive coupling and decoupling of stations or step wise commissioning of a system. Later extensions have no impact on stations that are already working on the bus.

When connecting the field devices it must be ensured that the data lines are not reversed. Always use a shielded data line to ensure high interference immunity of the system against electromagnetic emissions. The shield should be grounded on both sides and large-area shield clamps should be used for grounding to ensure good conductivity. Furthermore the data lines should be laid separately and away from all power cables. Never use spur lines for transmission rates > 1.5 Mbit/s.

A 9-pin D-Sub connector is primarily used to connect PROFIBUS devices on the bus, which complies with DIN 19245/ EN 50170. Normally connectors are used, which supports direct connection of the incoming and outgoing data cable.

This eliminates the need for spur lines and the bus connector can be connected and disconnected to the field device at any time without interrupting data communication. The type of connector suitable for RS485 transmission technology depends on the degree of protection.

To avoid signal reflections on the bus line, the line structure must be terminated by a bus termination resistor network at either end. The bus is terminated at each end of a segment with an active bus termination. Both bus terminators have a permanent power supply to ensure error-free operation. The bus terminator is usually switched in the devices or in the connectors.

RS485 installation recommendation

Problems with data transmission in PROFIBUS networks can be attributed to incorrect wiring or installation. These problems can often be solved using bus test equipment such as bus monitors, which are able to detect many typical wiring errors even before commissioning.

The list below helps to build up a RS485 network quick and accurate:

• Only use shielded and twisted-pair wiring as a RS485 bus line.

• Use a PROFIBUS bus connector plug to connect the RS485 bus segment.

• If the module is at the beginning or end of a bus segment, this connector must have an activated bus terminal resistor combination.

• Ensure that the bus segment connected to the RS485 interface is terminated at both ends.

• All PROFIBUS bus connector plugs in a network must be securely screwed onto the RS485 interfaces.

• Attaching or removing the bus connector plugs, inadequately attached bus connector plugs or loose bus wires within the plug can lead to malfunctions in the networks.

• Attach or remove the RS485 bus connector plug quickly and without twisting them.