BITBUS – a field bus with enormous expansion possibilities!
BITBUS is the technical name for a non-proprietary and open field bus. Here, the labelling as proprietary defines the delimitation from the characteristic as free. The meaning of this adjective is “property”. The first time BITBUS appeared as a component in the field of industrial communication and information technology was when it was introduced by Intel. This company specified the term in the year 1991. However, at that time the field bus was published under the name “IEEE 1118”. Under this name, the field bus established an international standard. The basic operation is divided into two different standards:
- RS485 interface – this interface serves to connect of different devices under a physical aspect.
- Software side – this standard uses synchronous data link control (SDLC), the synchronous data transmission control unit. This transmits the data transparently and biserially.
The characteristics of BITBUS
The basic characteristics of the field bus include the low TCO (total cost of ownership), the reliability, compatibility with many operating systems and an optimal ratio of the distance with regard to the speed that is reached with the transmission of data. In addition, BITBUS is characterised by a deterministic behavior. This is an algorithm which specifies that only clearly defined conditions occur, which are reproduceable at any time. This means that for the same form of input, the result is also always the same result as the exact statement. In addition, the same sequence of all conditions is passed through. This also has the consequence that the collected intermediate results always show the same values. In addition, with BITBUS there is a delegation of the real-time control into the field.
- The connection length and connection speed – the realization of the speed of the data transmission is connected with the length of the bus connection. For example, when there is a bus length of 300 m, a max. speed of 375 kbps can be reached. When the distance over which the field bus extends increases fourfold, the transmission rate is reduced to a value of approximately 62.5 kbps.
- Wiring – the wiring of the BITBUS takes place in accordance with the specification of the RS485 interface. However, the plug assignment is not defined at this point, but must be performed according to a specified tabulated classification. The first two pins are not assigned. The third pin is assigned DATA B (-). Optionally, the fourth pin can be plugged with RTS B (-). The fifth pin is assigned signal earth. The sixth and the seventh pin again are not assigned again. The positive components of DATA A and RTS A are found on the last two pins plug points. A twisted pair cable is used to connect the devices. The DATA A lines and the DATA B lines are twisted to form a twisted pair. In addition, the RTS A connection and the RTS B connection are also joined together to form such a pair. However, these pairings are only required for segments positioned far away from the repeater. The use of wiring as twisted wire pair is only required when the repeater also uses it. With the wiring of the BITBUS, the shielding is done by the signal earth.
The coding of the BITBUS
With this field bus, two different methods are available for coding the data.
- “Synchronous mode” – an additional wire pair is required to enable a transmission with this technique. After the connection, this transmits a synchronous impulse.
- “Self-clock mode” – with this coding method, the NRZI standard is used for data transmission. However, this also eliminates many repeaters and type LWL converters as they are no longer compatible due to the change from the original NRZ standard to the standard that already been mentioned.
The topology of the BITBUS
The max. number of participating devices within a single segment from one bus topology is 28. However, this can be expanded by connecting a compatible repeater. This increases the number of possible participants to 250. The use of at least two repeaters causes a reduction in the data rate to 62.4 kbps. However, after minimizing the transmission speed, it is possible to connect up to 10 coupled repeaters without further losses. Here, the length of the bus lines is either at 300 m or 1200 m. With the coupling of repeaters within a BITBUS, it is also necessary to set up a connection resistance of 120 Ohm at each end of the line.
Addressing with the BITBUS
With this type of the field bus, the available address space is between 0 and 255 (max.). Each individual participant who is in this system has his own address with a number between 1 and 249. The other addresses, namely 0, 250, 251, 252, 253, 254, and 255 cannot be assigned. According to the specifications of the newer standard, i.e. designation as IEEE 1118, the position 255 is reserved for the broadcast address.