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Summary
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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).
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BOF Delimiter
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All telegrams have a BOF (beginning of frame) byte denoting
the beginning of the telegram. The BOF is always 0111 1110.
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ADR Target Addresses
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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.
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| 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 |
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Message Field
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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.
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FCS (Frame Check Sequence)
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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.
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EOF (End of Frame) Delimiter
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All telegrams have an EOF byte denoting the end of the telegram.
The EOF is always 0111 1110.
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Data Record
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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.
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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.
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Master Synchronization Telegram
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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.
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Field
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Bytes
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Description
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BOF
ADR
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1
1
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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
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The lower 3 bits designate
the SERCOS phase (0 - 4) |
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FCS
EOF
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Frame check sequence. The FCS field contains circular
redundancy check (CRC) information.
End of Frame. The EOF marks the end of the telegram.
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Master Data Telegram
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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).
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Field
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Size (bytes)
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Description
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| BOF |
1
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Beginning of Frame, which is
always 0111 1110. The BOF marks the start of a telegram. |
| ADR |
1
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Target address.
In the MDT, ADR = 255 (the broadcast address). |
| *Control
Word |
2
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Control word for drive n. Contains
operational data. |
| *Service
Info |
2
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Contains the non-cyclic data
for drive n. |
| *IDN
Commands |
Variable **
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Contains the cyclic data for
drive n. |
| FCS |
2
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Frame check sequence. Contains
circular redundancy check (CRC) info. |
| EOF |
1
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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
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Bit
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Name & Value
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More Detail
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15
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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. |
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14
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0 Drive Disable
1 Drive Enable |
When 1->0, torque is immediately
disabled, independent of bits 15 and 13. |
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13
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0
Drive Halt
1 Drive Restart |
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12, 11
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Reserved |
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10
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Control
Unit Synchronization Bit |
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9, 8
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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 |
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7
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Real-time
Control Bit 2 |
S-0-302 |
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6
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Real-time Control
Bit 1 |
S-0-300 |
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5, 4, 3
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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. |
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0 Transmission
in progress
1 Last transmission |
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1
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0
Read service info
1 Write service info |
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0
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Master Service Transport Handshake
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A toggle bit |
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Amplifier Telegram
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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).
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Field
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Size (bytes)
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Description
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| BOF |
1
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Beginning of Frame.
The BOF marks the start of a telegram. |
| ADR |
1
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sends address. |
| Status |
2
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Control word for drive n. Contains
operational data. |
| Service Info |
2
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Contains the non-cyclic data
for drive n. |
| Operation Data |
Variable *
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Contains the cyclic data for
drive n. |
| FCS |
2
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Frame check sequence. Contains
circular redundancy check (CRC) info. |
| EOF |
1
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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.
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Bit
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Name & Value
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More Detail
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15, 14
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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 |
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13
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Drive
Shutdown Error, Class 3 Diags |
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12
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Change
Bit for Class 2 Diags |
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11
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Change
Bit for Class 3 Diags |
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9,8
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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 |
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7
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Real-time
Status Bit 2 |
See S-0-306
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6
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Real-time
Status Bit 1 |
See S-0-304 |
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5
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Change
Bit Commands
0 No Change in Command Status
1 Changing Command Status |
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4, 3
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Reserved
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2
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Error 0 No Error
1 Error in Service Channel |
Error message is in drive's
Service Channel. |
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1
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Busy
0 Step Finished 1 Step in Progress |
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0
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Master Service
Transport Handshake |
A Toggle bit |
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