PB Fieldbus Protocol Utility

PROFIBUS Telegram Decoder

Decode PROFIBUS FDL telegrams from hexadecimal bytes. Identify SD1, SD2, SD3, SD4 and short-acknowledgement frames, inspect station addresses, function control, SAP extensions, payload length, Frame Check Sequence and end delimiter.

✓ SD1 ✓ SD2 ✓ SD3 ✓ SD4 ✓ FCS ✓ SAP

PROFIBUS Telegram Decode

● Ready

PROFIBUS Telegram Hex Bytes Enter one complete PROFIBUS telegram. Spaces, commas, colons, hyphens, 0x-prefixed bytes and continuous hexadecimal strings are accepted.

Structural scope: this decoder handles PROFIBUS FDL telegram framing. It can expose raw service access point bytes when address-extension bits indicate their presence, but application meanings such as PROFIBUS-DP parameter, diagnostic and cyclic I/O fields depend on the higher-layer service and device profile.

PROFIBUS Telegram Decode Result Decoded

Decoded Telegram —

Frame Type —

Start Delimiter —

Destination Address —

Source Address —

DA Extension —

SA Extension —

Function Control —

FC Code —

Payload Length —

DSAP —

SSAP —

FCS —

Calculated FCS —

FCS Status —

End Delimiter —

Total Bytes —

User / Application Data

—

Telegram Breakdown

—

What Is a PROFIBUS Telegram?

PROFIBUS uses defined telegram formats at its Fieldbus Data Link layer to move control information and application data between bus stations. Different start delimiters identify different frame structures, allowing a receiver to determine whether a telegram has fixed length, variable length, data bytes, token information or only a short acknowledgement.

This decoder works from the hexadecimal telegram bytes themselves and exposes the framing information without requiring a GSD file or particular PLC configuration.

Main PROFIBUS Start Delimiters

Delimiter	Name	Structure
0x10	SD1	Fixed-length telegram without user data
0x68	SD2	Variable-length telegram
0xA2	SD3	Fixed-length telegram with 8 data bytes
0xDC	SD4	Token telegram
0xE5	SC	Short acknowledgement

PROFIBUS SD1 Telegram

SD1 begins with hexadecimal 0x10 and contains no user-data field. A normal SD1 telegram contains destination address, source address, Function Control, Frame Check Sequence and the end delimiter.

SD1:

10
DA
SA
FC
FCS
16

The FCS is calculated over the bytes from DA through FC.

PROFIBUS SD2 Variable-Length Telegram

SD2 is the variable-length PROFIBUS frame and begins with 0x68. It contains two identical length bytes and then repeats the SD2 delimiter before the address and control fields.

SD2:

68
LE
LEr
68
DA
SA
FC
DATA...
FCS
16

The two length fields must match. The repeated start delimiter must also equal 0x68.

How PROFIBUS SD2 Length Is Calculated

The SD2 length field counts the bytes beginning with Destination Address and ending with the final data byte.

LE includes:

DA
SA
FC
DATA bytes

Therefore:

Data Length =
LE - 3

FCS and the final 0x16 delimiter are not included in LE.

PROFIBUS SD3 Telegram

SD3 uses start delimiter 0xA2 and has a fixed eight-byte user-data field.

SD3:

A2
DA
SA
FC
DATA1
DATA2
DATA3
DATA4
DATA5
DATA6
DATA7
DATA8
FCS
16

The complete SD3 telegram therefore contains 14 bytes.

PROFIBUS SD4 Token Telegram

The SD4 token telegram is identified by hexadecimal 0xDC. It carries the destination and source station address used during token passing.

SD4:

DC
DA
SA

It does not use the normal FCS and 0x16 ending structure of SD1, SD2 and SD3.

PROFIBUS Short Acknowledgement

A single hexadecimal byte 0xE5 represents the short confirmation or acknowledgement telegram.

E5

Total length:
1 byte

Destination and Source Address

PROFIBUS telegrams contain DA and SA address bytes. The lower seven bits provide the station-address portion while the high bit can indicate that an address extension is being used.

Address byte:

bit 7
Address-extension indication

bits 0–6
Station address portion

This calculator therefore displays both the raw address byte and the lower seven-bit station value rather than silently discarding the extension flag.

DSAP and SSAP

When PROFIBUS address-extension mechanisms are in use, additional destination and source Service Access Point bytes can occur at the beginning of the data portion.

The decoder separates DSAP and SSAP only when the corresponding DA and SA extension bits indicate their presence and enough payload bytes are available. The remaining bytes are then displayed as user/application data.

PROFIBUS Function Control Byte

The Function Control, or FC, byte contains control bits and a lower four-bit function code. Some upper control-bit meanings depend on whether the telegram is a primary request or secondary response and on the particular FDL service.

FC lower nibble:

FC & 0x0F

= Function Code

The calculator exposes the complete hexadecimal FC value, individual upper bits and the four-bit function code without assigning a higher-layer service that cannot be proven from the frame alone.

PROFIBUS Frame Check Sequence

The PROFIBUS FCS is an eight-bit arithmetic checksum. Each protected byte is added and only the low eight bits of the result are retained.

FCS =
sum(protected bytes) mod 256

For SD1 the protected bytes are DA, SA and FC. For SD2 and SD3, user-data bytes are included as well.

PROFIBUS End Delimiter

SD1, SD2 and SD3 telegrams normally terminate with hexadecimal 0x16. The decoder verifies that this byte is present at the expected position after the FCS.

... FCS 16

16 = End Delimiter

PROFIBUS DP and the FDL Telegram

PROFIBUS-DP services are carried using these underlying telegram structures, but an FDL frame alone does not necessarily describe the engineering meaning of each user byte. Cyclic input/output data, diagnostics, parameterization, configuration and acyclic services use additional protocol conventions.

For that reason, this decoder concentrates on the telegram framing and preserves application data in hexadecimal form rather than applying an unrelated device layout.

PROFIBUS Telegram Decoder FAQs

What does PROFIBUS start delimiter 0x10 mean?

0x10 identifies an SD1 fixed-length telegram without a user-data field.

What does PROFIBUS start delimiter 0x68 mean?

0x68 identifies the SD2 variable-length telegram format.

What is SD3?

SD3 is a fixed-format telegram beginning with 0xA2 and carrying eight user-data bytes.

What is SD4?

SD4 is the token telegram beginning with 0xDC.

What does 0xE5 mean in PROFIBUS?

A single 0xE5 byte is the short acknowledgement telegram.

How is the SD2 data length calculated?

The LE field counts DA, SA, FC and data. Therefore the user-data portion has LE minus 3 bytes.

Does SD2 contain two length bytes?

Yes. LE is repeated and both values should match before the repeated 0x68 start delimiter.

How is the PROFIBUS FCS calculated?

The protected telegram bytes are added together and the low eight bits of the sum form the FCS.

What is the PROFIBUS end delimiter?

SD1, SD2 and SD3 normally use hexadecimal 0x16 as the end delimiter.

Can this decoder interpret PLC I/O values?

Not generically. The meaning of application bytes depends on the PROFIBUS service, device profile, configuration and often the device’s GSD information.

Decode PROFIBUS FDL Telegrams

Inspect PROFIBUS start delimiters, station addresses, Function Control, SAP extensions, payload boundaries, FCS checksums and end delimiters directly from raw hexadecimal telegrams.