Type A is especially recommended for high transmission speeds (>500 kBaud) and permits doubling of the network distance in comparison to Type B.

Technical specification:

Impedance: 35 up to 165 Ohm at frequencies from 3 to 20 Mhz.

Cable capacity: < 30 pF per meter.

Core diameter: > 0,34 mm², corresponds to AWG 22.

Cable type: twisted pair cable. 1×2 or 2×2 or 1×4 lines.

Resistance: < 110 Ohm per km.

Signal attenuation: max. 9 dB over total length of line section.

Shielding: CU shielding braid or shielding braid and shielding foil

Max. Bus length: 200 m at 1500 kbit/s, up to 1,2 km at 93,75 kbit/s. Extendable by repeaters.

The two types of messages used are request messages and confirmation messages. The request message causes an action in a task and the confirmation message confirms the performing of that action or task (answer).

For the transmitter to send the message it needs to write it into the receiver’s mailbox and notify the receiver by inverting one of its mailbox bits. The receiver then reads the mail and releases the mailbox in response to the corresponding mail. Only one mailbox is used in each direction. The receiver of the message is thus responsible to quickly read the message in order to release the mailbox again for the next available message.

The messages that cannot immediately be transferred to the Host application are placed in a queue mechanism with a standard configuration of 31 available segments. The actual available segments can be read from the SegmentCount location inside the dual-port memory. If the available number of segments fall bellow a protocol dependent value, the device won’t be able to access messages from its mailbox. The Host application is thus responsible to read out its mailbox messages to ensure that no Host messages are in the queue witch will reduces the available segment count.

Synchronization of message delivery:

The procedure chosen for message synchronization allows the Host to operate in interrupt mode as well as polling mode. The device can cause an interrupt to the Host application by writing into the interrupt capable handshake cell. The hardware has only one interrupt line for each handshake register, indicating different events. The event can be identified by reading the corresponding handshake register in the dual-port memory. The two reserved handshake registers in the duel-port memory are the DevFlags and HostFlags registers. The DevFlags register can only be written to by the Host application and read from by the device. The HostFlags register on the other hand can only be written to by the device and read from by the Host application.

Both these registers contain a command and acknowledgement bit. A command gets activated when the command and acknowledgement bit has opposite values and acknowledged when they have the same value. It is important to note that a new command can only be activated if the preceding command was acknowledged.

In polling mode the Host application monitors the difference between the command and acknowledgement flag bits. The Host application will then be able to send messages or received messages depending on the status of these bits.

PROFIBUS is divided into two variations, the more commonly used DP (Decentralized Peripheral) version that replaced the first complex communication protocol version FMS (Fieldbus Message Specification) in 1993 and the less commonly used PA (Process Automation) protocol version.

PROFIBUS DP is the high speed solution of PROFIBUS. It has been designed and optimized especially for communication between automation systems and decentralized devices. It can operate at data rates of up to 12Mbit/s over twisted pair cables or fiber optic links. The communication is via cyclic data traffic exclusively and each field device need to exchange its data with the control device (Master) within a given cycle time. Operation and monitoring tasks for a visualization device separate from the control device was not supported by this specification and appropriate function extensions were defined in 1997. The new extended version is referred to as PROVIBUS-DPV1 and is compatible with all earlier versions of the existing DP protocol. PROVIBUS-DPV1 also offers in addition to the cyclic DP communication services, an acyclic service for alarm messages, diagnostics and control of field devices.

PROFIBUS AP is used to monitor measuring equipment via a process control system. It also meets the two special requirements of process automation, an intrinsically safe transmission technique and field device power over bus cable. The disadvantage of this protocol is its slow data rate of only 31.25kbit/s. Weak current flow through the bus lines makes it intrinsically safe and ideal for use in explosion-hazardous areas.

The FROFIBUS protocol (OSI reference model)

User Interface:

For the User Interface of PROFIBUS various service levels were defined for the DP protocol:

· DP-V0 provides the service for cyclic exchange of data and diagnosis.

· DP-V1 provides the service for acyclic and cyclic exchange and alarm handling.

· DP-V2 provides the service for isochronous mode data exchange broadcast (slave-to-slave communication).

Data Link layer:

The Field Bus Data Link Layer (FDL) work with a hybrid bus access protocol. It combines token passing with a master slave method. The token passing procedure guarantees that the bus access gets assigned to a specific master node for a defined time frame. In this time frame the master node can access the bus and request data from any master or slave device. After the time frame expires the current master node will pass the token to the next master node on the bus. This layer also includes the handling of data security and the transmission protocol of telegrams.

Physical layer or bit-transmission layer:

Three different methods are specified for the physical layer of FROFIBUS

· Electrical wire transmission EIA-485, twisted wire pair with impedance of 150 ohms. Specified for Bit rates 9.6 kbit/s to 12Mbit/s. This transmission method is usually used with PROFIBUS DP. The cable length between repeaters is limited from 100 to 1200 m, depending on the bit rate.

· Fiber optical transmission uses, star-, bus- and ring topologies and the distance between repeaters can be up to 15 km.

· With Manchester Bus Powered transmission technology the data and device power are fed through the same cable. The bit rate is a fixed 31.25 kbit/s. This technology was specifically developed for PROFIBUS PA.

Other transmission mediums products like radio waves, sliding contacts etc. can be obtained from various manufacturers but this does not mean they conform to any standards.