Sercos- Telegrams

Summary

The controller (Master) communicates with the drives and I/O modules (Slaves) using telegrams. A telegram is a structure that contains data, error checking and handshaking information. SERCOS supports three types of telegrams: Master Synchronization Telegram (MST), Master Data Telegram (MDT), and Amplifier Telegram (AT).

Each type of telegram contains 5 types of fields: BOF (beginning of frame), ADR (address), message field, FCS (frame check sequence), EOF (end of frame).

 

BOF Delimiter

All telegrams have a BOF (beginning of frame) byte denoting the beginning of the telegram. The BOF is always 0111 1110.

 

ADR Target Addresses

All telegrams have an ADR (address) byte, which denotes the address of a drive (Slave). In a telegram from the Master, the address specifies which drive the information is for. In a telegram from a Slave (drive), the address specifies the sourcing drive. The target addresses for the drives are valid if greater than 0 and less than 255. Typically, a drive's address is set using a selector located physically on the drive.

Address 0 is the "no station" address, and is sometimes used to remove a drive from the ring logically, during troubleshooting. During non-cyclic operations (Phases 0 - 2), the Master can only communicate with one drive per cycle. During cyclic operations (Phases 3, 4), the Master can communicate with all drives.

From To Telegram Non-Cyclic
(Phases 0,1,2)
Cyclic
(Phases3,4)
Master Slave MST 255 255
Master Slave MDT 1 < ADR < 255 255
Slave Master AT 1 < ADR < 254 1 < ADR < 254
 

Message Field

Each Telegram (MST, AT, and MDT) contains a message field. The Message Field for the MST consists of one 8-bit word. The lower three bits of the MST contain Phase information. The Message Field for the AT consists of one Data Record. The Message Field for the MDT consists of one Data Record if the MDT's target address is a specific drive. If the MDT's Target address is all drives (255), then the Message Field for the MDT consists of one Data Record per drive in the system.

 

FCS (Frame Check Sequence)

All telegrams have a two-byte FCS number used to check data integrity. The frame check sequence (16 bits) is implemented according to ISO/IEC 3309, 4.5.2.

 

EOF (End of Frame) Delimiter

All telegrams have an EOF byte denoting the end of the telegram. The EOF is always 0111 1110.

 

Data Record

Data Records are used by both the Amplifier Telegram (AT), and the Master Data Telegram (MDT) to send data. Generally, a Data Record consists of a 16-bit Control/Status word, a 16-bit Service Channel, and a Data Block of 16-bit words.

Amplifier Telegrams contain only one Data Record, because each Slave sends its own Amplifier Telegram.

During Phase 2, the Master sends a Master Data Telegram that is addressed to a specific Slave. Since only one Slave is to receive the MDT, the Phase 2 MDT has only one Data Record. During Phases 3 and 4, the Master sends a Master Data Telegram that is received by all Slaves on the ring. Since all Slaves receive the Master Data Telegram, the Phase 3-4 MDT has one Data Record for each Slave.

During Phase 2, the Data Block in both the AT and the MDT has a length of 0. This means that all data exchange between Master and Slave must take place using the Service Channel. Because the Service Channel is only 16-bits wide, data exchange can take multiple cycles. Service Channel exchange of data is referred to as non-cyclic.

During Phase 2, the Master is responsible for determining the data fields for the Data Blocks in the Phase 3-4 AT and MDT. The size and number of data fields will determine the size of the Data Block. Once defined in Phase 2, the fields and therefore the size of the Data Block is fixed. The data fields in the MDT usually contain command data. The data fields in the AT usually contain feedback data. Because the Data Blocks are configured in Phase 2, there is no overhead using the Data Blocks to send data in Phases 3 and 4. Data exchange using the Data Blocks is referred to as cyclic.

Master Synchronization Telegram

The Master Controller uses master synchronization telegrams (MST) to coordinate its transmission cycle timing with the Slaves. The Master initiates a SERCOS cycle by transmitting an MST to all of the drives and I/O modules on the ring.The MST message field contains one 8-bit word, of which the three lowest bits determine the communication phase of the system. The MST is sent at the beginning of the SERCOS cycle in all communication phases.

Field
Bytes
Description

BOF
ADR

1
1
Beginning of Frame. The BOF marks the start of a telegram.
The target address. In the MST, ADR = 255 (the broadcast address) during Phases 3 and 4.
Current Phase
1
The lower 3 bits designate the SERCOS phase (0 - 4)

FCS

EOF

2

1

Frame check sequence. The FCS field contains circular redundancy check (CRC) information.
End of Frame. The EOF marks the end of the telegram.

 

Master Data Telegram

During Phase 2, the Master must communicate with the Slaves in order to configure them for operations in Phases 3 and 4. In order to send or request data, the Master will send a Master Data Telegram (MDT) to a specific Slave (the Slave is addressed explicitly). Since only one Slave is to receive the MDT, the MDT Message Field contains only one Data Record, where the length of the Data Block (inside the Data Record) is 0, i.e., the data block is empty. During Phase 2, the Master sends data to each Slave via the Service Channel.

During Phase 2, the Master informs the Slave of the byte offset into the MDT at which the Data Record resides for that Slave. The length (in bytes) of the Data Block within each Data Record depends on the data fields configured by the Master during Phase 2. The data fields within the Data Block usually contain command information.

During Phases 3 and 4, once per SERCOS cycle the Master sends a Master Data Telegram (MDT) that has a global (broadcast) address. Because the global address is used, all Slaves receive the MDT. Since all Slaves will receive the MDT, the MDT Message Field contains one Data Record for each Slave in the system. During Phases 3 and 4, data is sent to a Slave by using the Data Blocks (cyclic) or the Service Channel (non-cyclic).

Field
Size (bytes)
Description
BOF
1
Beginning of Frame, which is always 0111 1110. The BOF marks the start of a telegram.
ADR
1
Target address. In the MDT, ADR = 255 (the broadcast address).
*Control Word
2
Control word for drive n. Contains operational data.
*Service Info
2
Contains the non-cyclic data for drive n.
*IDN Commands
Variable **
Contains the cyclic data for drive n.
FCS
2
Frame check sequence. Contains circular redundancy check (CRC) info.
EOF
1
End of Frame, which is always 0111 1110. The EOF marks the end of the telegram.
* These fields comprise the data record. There is one data record per drive in the MDT.
** The length if the IDN Commands field is determined during Phase 2
Bit
Name & Value
More Detail
15
0 Drive OFF
1 Drive ON
Bit 15-13=111, the drive should follow command values.
When 1->0, the drive removes torque from the motor, and allows the motor to spin down.
14
0 Drive Disable
1 Drive Enable
When 1->0, torque is immediately disabled, independent of bits 15 and 13.
13
0 Drive Halt
1 Drive Restart
 
12, 11
Reserved  
10
Control Unit Synchronization Bit  
9, 8
Operation Mode
00 primary op mode
01 secondary op mode 1
10 secondary op mode 2
11 secondary op mode 3
Defined byS-0-32
Defined by S-0-33
Defined by S-0-34
Defined by S-0-35
7
Real-time Control Bit 2 S-0-302
6
Real-time Control Bit 1 S-0-300
5, 4, 3
Data Block Element
000 Service channel not active,
001 IDN (number) of the op data
010 Name of operation data
011 Attribute of op data
100 Units of op data
101 Min input value
110 Max input value
111 Operation data
-Close service channel or break a transmission in progress -The service channel is closed for the previous IDN and opened for a new IDN.
2
0 Transmission in progress
1 Last transmission
 
1
0 Read service info
1 Write service info
 
0
Master Service Transport Handshake
A toggle bit
 

Amplifier Telegram

During Phase 2, the Slave sends an Amplifier Telegram (AT) only when it receives a Master Data Telegram (MDT) that contains that Slave's address. The AT contains one Data Record where the length of the Data Block is 0, i.e., the data block is empty. During Phase 2, the Master sends data to each Slave via the Service Channel.

During Phases 3 and 4, each Slave sends an Amplifier Telegram (AT) every SERCOS cycle, at the time designated by the Master during Phase 2. The AT Message Field contains one Data Record, where the length of the Data Block (inside that Data Record) is determined by the data fields configured by the Master during Phase 2. The data fields within the Data Block usually contain feedback and status information.

During Phases 3 and 4, data is sent to the Master by using the Data Blocks (cyclic) or the Service Channel (non-cyclic).

Field
Size (bytes)
Description
BOF
1
Beginning of Frame.
The BOF marks the start of a telegram.
ADR
1
sends address.
Status
2
Control word for drive n. Contains operational data.
Service Info
2
Contains the non-cyclic data for drive n.
Operation Data
Variable *
Contains the cyclic data for drive n.
FCS
2
Frame check sequence. Contains circular redundancy check (CRC) info.
EOF
1
End of Frame, which is always 0111 1110. The EOF marks the end of the telegram.
* The length of the Operation Data field is determined during Phase 2.

Bit
Name & Value
More Detail
15, 14
Ready to operate
00 drive not ready for power-up
01 drive ready for power-up
10 drive power ready
11 drive ready to operate
 
13
Drive Shutdown Error, Class 3 Diags  
12
Change Bit for Class 2 Diags  
11
Change Bit for Class 3 Diags  
9,8
Actual Operation Mode
00 primary op mode
01 secondary op mode 1
10 secondary op mode 2
11 secondary op mode 3

Defined by S-0-32
Defined by S-0-33
Defined by S-0-34
Defined by S-0-35
7
Real-time Status Bit 2 See S-0-306
6
Real-time Status Bit 1 See S-0-304
5
Change Bit Commands
0 No Change in Command Status
1 Changing Command Status
 
4, 3
Reserved  
2
Error 0 No Error 1 Error in Service Channel Error message is in drive's Service Channel.
1
Busy 0 Step Finished 1 Step in Progress  
0
Master Service Transport Handshake A Toggle bit