Siemens SPC3 Specifications download pdf (Page 36)

Introduction To ProfiBus DP

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Acromag, Inc. Tel:248-624-1541 Fax:248-624-9234 Email:sales@acromag.com http://www.acromag.com

Unlike CAN and Ethernet which are event-driven busses, ProfiBus was

designed to guarantee a deterministic response.. The

determinism

of a

system refers to the ability to precisely predict the behavior of the system

over time.

ProfiBus uses a polling mechanism between master and slave. The

time it takes a slave to respond to a message from the master is the

reaction

time

. Even if a ProfiBus system receives many I/O signal changes at some

point in time, there is no change in reaction time. Further, even if another

master (class 2) is used to perform diagnostics on a slave device while it is

communicating with its class 1 master, the reaction time for the system will

remain the same. This is because the class 2 master used to perform

diagnostics will not be allowed to use more time than the configured gap

time within the bus cycle.

Because ProfiBus is deterministic, we can calculate a reliable system

reaction time. But before we get into the details of calculating bus cycle

times, we must define a few terms as follows:

Bit-Time:

To help simplify timing calculations, it is convenient to normalize

the time units with respect to the baud rate by using units of Bit-Time (Tbit).

One bit-time is the time it takes to transmit one bit and is the reciprocal of

the transmission rate (baud rate). For example:

1 Tbit (Bit Time) at 12MB = 1/12000000bps = 83ns/bit

Sync-Time (T

SYN

The synchronization time is the minimum time a station

must remain in the idle state before it can accept another request. For

ProfiBus DP, an idle state of 33Tbits (bit-time) must be present before every

request telegram and this is called the sync-time.

Slave Reaction Time (T

SDR

The reaction time is the time it takes a slave

to respond to a message. This time is often expressed as a minimum value

(min T

SDR

), or maximum value (max T

SDR

). Min T

SDR

is set within the

parameterization telegram during startup. Max T

SDR

varies with the

transmission rate and is specified at the supported baud rates within the

device GSD file. For ProfiBus DP, this value may range from a minimum of

11Tbits (min T

SDR

default) to a maximum of 255Tbits.

Initiator Delay Time (T

SDI

refers to the station delay of the

initiator

a request or token frame (the master).

Initiator Idle Time (T

ID1

After receiving the last character of a telegram,

the initiator must wait this amount of time until it sends the next telegram.

The idle time (Tid1) is the time between transmission of the last bit of a

frame (no acknowledge) and the transmission of the first bit of the next

frame. It is at least the sync time (T

SYN

), plus some safety margin (Tsm), but

is also calculated as the maximum of these three values: T

SYN

+ Tsm, min

SDR

, or T

SDI

(station delay of telegram initiator). The addition of safety

margin (Tsm) is very important at high baud rates.

BUS TIMING

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Siemens SPC3 Specifications Page 36

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