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4.5 Frame Structure

4.5.1 Frame Character (UART Character)

Each frame consists of a number of frame characters, the UART characters. The

UART character (UC) is a start-stop character for asynchronous transmission and

is structured as follows:

Transmitted

Bit Sequence 1st 2 3 4 5 6 7 8 9 10 11th

Information 2

0

2

7

sig. Bits LSB MSB

---+ +---+---+---+---+---+---+---+---+---+---+---

! 0 ! b1! b2! b3! b4! b5! b6! b7! b8! P ! 1 !

+---+---+---+---+---+---+---+---+---+---+

Start bit----! ! ! ! !---Stop bit

(ST) ! Octet ! ! (SP)

+<----------------------------->+ !

!-----Parity bit

even

Figure 9. UART Character

The presentation of UART characters is based on the following standards: ISO

1177, ISO 2022.

Transmission Rule

Each UART character consists of 11 bits: a start bit (ST) which is always binary

"0", 8 information bits (I) which may be binary "0" or binary "1", an even par-

ity bit (P) which is binary "0" or binary "1" and a stop bit (SP) which is

always binary "1".

4.5.2 Bit Synchronizing

The receiver's bit synchronizing always starts with the falling edge of the

start bit, i.e. at the transition from binary "1" to binary "0". The start bit

and all consecutive bits are scanned in the middle of the bit time. The start

bit shall be binary "0" in the middle of the bit, otherwise the synchronizing

fails and is stopped. The synchronizing of the UART character ends with the stop

bit being binary "1". If a binary "0" bit is encountered instead of the stop

bit, a synchronizing error or UART character error is assumed and reported and

the next leading edge of a start bit is waited for.

A maximum deviation of ± 0,3 % of the nominal signalling rate for transmission

and receipt is allowed (see Part 2, subclause 4.1.1, Data Signalling Rate).

4.6 Frame Formats

The figures contained in the following clauses do not show sequences (request →

acknowledgement or response), but frame formats of the same category (Hd = 4,

fixed length without/with data field and variable length), i.e. the action

frames may be followed by different acknowledgement or response frames (see

clause 4.8).

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4.6.1 Frames of fixed Length with no Data Field

A) Format of the Request Frame:

+-/ /-+----+----+----+----+----+----+

! SYN !SD1 ! DA ! SA ! FC !FCS ! ED !

+-/ /-+----+----+----+----+----+----+

! !

! L !

+<------------>+

B) Format of the Acknowledgement Frame:

+----+----+----+----+----+----+

!SD1 ! DA ! SA ! FC !FCS ! ED !

+----+----+----+----+----+----+

! !

! L !

+<------------>+

C) Format of the Short Acknowledgement Frame:

+----+

! SC !

+----+

where:

SYN Synchronization Period, a minimum of 33 line idle bits

SD1 Start Delimiter, value: 10H

DA Destination Address

SA Source Address

FC Frame Control

FCS Frame Check Sequence

ED End Delimiter, value: 16H

L Information Field Length, fixed number of octets: L = 3

SC Single Character, value: E5H

Figure 10. Frames of fixed Length with no Data Field

Transmission Rules

1. Line idle state corresponds to signalling level binary "1".

2. Each action frame shall be preceded by at least 33 line idle bits (Syn Time).

3. No idle states are allowed between a frame's UART characters.

4. The receiver checks:

per UART character: start bit, stop bit and parity bit (even), per frame:

start delimiter, DA, SA, FCS and end delimiter and the SYN Time in the case

of an action frame. If the check fails, the whole frame shall be discarded.

SC and SD1 (as well as SD2 and SD3, see subclauses 4.6.2 and 4.6.3) have Hamming

distance Hd=4 and are safe against being shifted (see IEC 870-5-1), i.e. the

single character SC appears to be a frame with Hd=4.

For requests to be acknowledged only (Send Data with Acknowledge), SC is a per-

missible positive acknowledgement. For requests to be answered (Send and Request

Data with Reply), SC is a permissible negative acknowledgement if no data is

available (see table 3a, b7=0, Code-No 9).

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4.6.2 Frames of fixed Length with Data Field

A) Format of the Send/Request Frame:

+-/ /-+----+----+----+----+-----------+----+----+

! SYN !SD3 ! DA ! SA ! FC ! DATA_UNIT !FCS ! ED !

+-/ /-+----+----+----+----+-----------+----+----+

! !

! L !

+<------------------------>+

B) Format of the Response Frame:

+----+----+----+----+-----------+----+----+

!SD3 ! DA ! SA ! FC ! DATA_UNIT !FCS ! ED !

+----+----+----+----+-----------+----+----+

! !

! L !

+<------------------------>+

where:

SYN Synchronization Period, a minimum of 33 line idle bits

SD3 Start Delimiter, value: A2H

DA Destination Address

SA Source Address

FC Frame Control

DATA_UNIT Data Field, fixed Length (L-3) = 8 octets

FCS Frame Check Sequence

ED End Delimiter, value: 16H

L Information Field Length,

fixed number of octets: L = 11

Figure 11. Frames of fixed Length with Data Field

Transmission Rules

The same transmission rules as for frames of fixed length with no data field are

valid here (see subclause 4.6.1).

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4.6.3 Frames with variable Data Field Length

For a variable number of data octets the length shall also be transmitted in the

frame. This length information is contained twice in a fixed frame header at the

beginning of the frame. Thus it is protected with Hd = 4 and safe against slip.

A) Format of the Send/Request Frame:

+-/ /-+----+----+----+----+----+----+----+----/ /----+----+----+

! SYN !SD2 ! LE ! LEr!SD2 ! DA ! SA ! FC ! DATA_UNIT !FCS ! ED !

+-/ /-+----+----+----+----+----+----+----+----/ /----+----+----+

! !

! L !

+<------------------------>+

B) Format of the Response Frame:

+----+----+----+----+----+----+----+----/ /----+----+----+

!SD2 ! LE ! LEr!SD2 ! DA ! SA ! FC ! DATA_UNIT !FCS ! ED !

+----+----+----+----+----+----+----+----/ /----+----+----+

! !

! L !

+<------------------------>+

where:

SYN Synchronization Period, a minimum of 33 line idle bits

SD2 Start Delimiter, value: 68H

LE Octet Length, allowed values: 4 to 249

LEr Octet Length repeated

DA Destination Address

SA Source Address

FC Frame Control

DATA_UNIT Data Field, variable Length (L-3), max. 246 octets

FCS Frame Check Sequence

ED End Delimiter, value: 16H

L Information Field Length,

variable number of octets: L = 4 to 249

Figure 12. Frames with variable Data Field Length

Transmission Rules

The same transmission rules as for frames of fixed length with no data field are

valid (see subclause 4.6.1).

In addition to transmission rule 4, LE shall be identical to LEr, and the

information octets shall be counted from the destination address (DA) up to the

frame check sequence (FCS) and the result shall be compared with LE.

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4.6.4 Token Frame

+-/ /-+----+----+----+

! SYN !SD4 ! DA ! SA !

+-/ /-+----+----+----+

where:

SYN Synchronization Period, a minimum of 33 line idle bits

SD4 Start Delimiter, value: DCH

DA Destination Address

SA Source Address

Figure 13. Token Frame

Transmission Rules

1. Line idle state corresponds to signalling level binary "1".

2. Each token frame shall be preceded by at least 33 line idle bits (Syn Time).

3. No idle states are permitted between a frame's UART characters.

4. The receiver checks:

per UART character: start bit, stop bit and parity bit (even),

per frame: Syn Time, start delimiter and DA/SA. If the check fails, the whole

frame shall be discarded.

4.7 Length, Address, Control and Check Octet

4.7.1 Length Octet (LE, LEr)

The two length octets of identical value in the frame header of the variable

format contain the number of information octets in the frame body. These com-

prise: DA, SA, FC and the DATA_UNIT. The value covers the range from 4 to 249,

so that a maximum of 246 octets may be transmitted in a frame's DATA_UNIT (see

subclause 4.7.5). A value < 4 is not permitted, as a frame contains at least DA,

SA, FC and one DATA octet. The longest frame contains a total of 255 octets.

b8 b1

+---+---+---+---+---+---+---+---+

! 2

7

! ! 2

0

!

+---+---+---+---+---+---+---+---+

! !

! L !

+<----------------------------->+

L = 4 to 249

Figure 14. Length Octet Coding

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4.7.2 Address Octet (DA/SA)

The two address octets in the frame header (action, acknowledgement and response

frames) contain the destination (DA) and source (SA) station address. The token

frame consists only of these two address octets after the start delimiter.

b8 b1

+---+---+---+---+---+---+---+---+

!EXT! 2

6

! ! 2

0

!

+---+---+---+---+---+---+---+---+

! !

! Address !

+<------------------------->+

DA = 0 to 127 ; SA = 0 to 126

Figure 15. Address Octet Coding

Address 127 (b1 to b7 = 1) is reserved as global address for broadcast and mul-

ticast messages (frame to all stations or a group of stations selected by means

of a service access point; only permitted in Send Data with No Acknowledge,

SDN).

Thus, 127 station addresses (0 to 126) are available for slave and master sta-

tions, of which preferably no more than 32 may be occupied by master stations.

For non time-critical applications up to 127 master stations are optionally per-

mitted. As at least one master station is required, a maximum of 126 addresses

for slave stations is possible.

The action frame's address octets shall be sent back mirrored in the acknow-

ledgement or response frame, i.e. SA of the acknowledgement or response frame

contains the destination station address and DA contains the source station ad-

dress of the action frame.

Address Extension (EXT):

In frames with DATA_UNIT the EXT bit (extension) indicates a destination and/or

source address extension (DAE, SAE), which immediately follows the FC octet in

the DATA_UNIT. It may be distinguished between access address (Link Service Ac-

cess Point, LSAP, see subclause 4.7.2.2) and region/segment address. Both ad-

dress types may also occur simultaneously, as each address extension contains an

EXT bit again (see bit b8 in Fig. 17).

The address extensions of the action frame shall be sent back mirrored in the

response frame.

EXT = 0 : No address extension in the DATA_UNIT

EXT = 1 : Address extension follows in the DATA_UNIT

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EXT=1

EXT=0

-----+----+----+----+----+--------------/ /-------------+-----

! DA ! SA ! FC !DAE ! !

-----+----+----+----+----+--------------/ /-------------+-----

! !

! DATA_UNIT !

+<--------------------------------->+

EXT=0

EXT=1

-----+----+----+----+----+-------------/ /--------------+-----

! DA ! SA ! FC !SAE ! !

-----+----+----+----+----+-------------/ /--------------+-----

! !

! DATA_UNIT !

+<--------------------------------->+

EXT=1

-----+----+----+----+----+----+--------/ /--------------+-----

! DA ! SA ! FC !DAE !SAE ! !

-----+----+----+----+----+----+--------/ /--------------+-----

! !

! DATA_UNIT !

+<--------------------------------->+

Figure 16. DAE/SAE Octet in the Frame

b8 b7 b6 b1

+---+----+---+---+---+---+---+---+

!EXT!Type! 2

5

! ! 2

0

!

+---+----+---+---+---+---+---+---+

! !

! Address !

+<--------------------->+

b7 denote the Type:

0 6 bit Link Service Access Point (LSAP):

DAE = 0 to 63 ; SAE = 0 to 62

1 6 bit Region/Segment Address for Realization of hierarchical Bus Systems with

Bridges, Range of Values is to be defined.

b8 (EXT) denotes an additional address extension:

0 No additional address extension octet

1 One additional address extension octet follows

immediately

with the same

structure. The following order is valid:

First Octet : Region/Segment Address with b7=1, b8=1

Second Octet: LSAP with b7=0, b8=0

Figure 17. Address Extension Octet

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4.7.2.1 Address Check

A receiver checks the destination address information in a frame addressed to

itself by following rules with TS:

- Is there no Region/Segment address in the frame existent (DA[b8=0] or

DAE[b7=0]), it merely shall check the DA on equality.

- Is there a Region/Segment address in the frame existent (DAE[b7=1]), it shall

check the DA and the DAE on equality. Does TS of the receiver not contain a

Region/Segment address then the frame count not addressed to the receiver.

4.7.2.2 Link Service Access Point (LSAP)

At the FDL user - FDL interface (see chapter 5) a data transmission service (or

several thereof) is processed via a Link Service Access Point (LSAP). Several

LSAPs at the same time are permitted in master and slave stations. In this case

the related LSAP shall be transmitted together with the message.

The address extensions DAE and SAE are available for the transmission of LSAPs.

The Source Service Access Point (SSAP), which represents the access address of

the local user to the FDL, is transmitted in the SAE octet. The Destination

Service Access Point (DSAP), which represents one or all access addresses of the

remote user to the FDL, is transmitted in the DAE octet. SSAP values from 0 to

62 and DSAP values from 0 to 63 may be chosen. The DSAP value 63 (DAE b1 to

b6 = 1) represents the global access address. This DSAP is only allowed for the

send data services "SDA" and "SDN" (see Part 3, clause 4).

If the transmission of LSAPs is omitted for reasons of frame efficiency, the

data transmission services shall be processed via the Default LSAP. In this case

all action frames are transmitted without SAE. All correctly received acknow-

ledgement or response frames without DAE are assigned to this default LSAP. At

the FDL user - FDL interface (see Part 3, clause 4.1) the default LSAP is manda-

tory and is addressed with the value NIL.

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4.7.3 Control Octet (FC)

The control octet in the frame header indicates the frame type, such as action

frame (request or send/request frame) and acknowledgement or response frame. In

addition the control octet contains the function and the control information,

which prevents loss and multiplication of messages, or the station type with the

FDL state.

b8 b7 b6 b5 b4 b1

+-----+-----+-----+-----+-----+-----+-----+-----+

! ! 1 ! FCB ! FCV ! 2

3

2

0

!

! Res !Frame!-----+-----+-- Function --!

! ! 0 ! Stn-Type ! !

+-----+-----+-----+-----+-----+-----+-----+-----+

Res

: Reserved

(the sender shall be set binary "0",

the receiver does not have to interpret)

Frame

Type: 1 Request, Send/Request Frame

0 Acknowledgement, Response Frame

b7 = 1:

FCB

Frame Count Bit: 0/1, alternating

FCV

Frame Count Bit valid:

0 alternating Function of FCB is invalid

1 alternating Function of FCB is valid

b7 = 0:

Stn-Type

: Station Type and FDL Status

b6 b5 <-- Bit Position

0 0 Slave Station

0 1 Master Station not ready to enter logical token ring

1 0 Master Station ready to enter logical token ring

1 1 Master Station in logical token ring

Function:

see Table 3a

Figure 18. FC Octet Coding

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Table 3a. Transmission Function Codes

+-------+--------------------------------+----------------------+

! Code ! Function ! Format !

! No ! ! in Clause !

+-------+--------------------------------+----------------------+

! ! Frame Type b7 = 1 ! !

! ! ! !

! 0,1,2 ! Reserved

! !

! 3 ! Send Data with Acknowledge low ! 4.6.2/3A !

! 4 ! Send Data with No Acknow. low ! - " - !

! 5 ! Send Data with Acknowledge high! - " - !

! 6 ! Send Data with No Acknow. high ! - " - !

! 7 ! Reserved (Req. Diagnosis Data) ! !

! 8 ! Reserved

! !

! 9 ! Request FDL Status with Reply ! 4.6.1A !

! 10,11 ! Reserved ! !

! 12 ! Send and Request Data low ! 4.6.1/2/3A !

! 13 ! Send and Request Data high ! - " - !

! 14 ! Request Ident with Reply ! 4.6.1A !

! 15 ! Request LSAP Status with Reply ! 4.6.1/3A !

! ! (Code No 14 and 15: FMA1/2) ! !

+-------+--------------------------------+----------------------+

! ! Frame Type b7 = 0 ! !

! ! ! !

! 0 ! ACKnowledgement positive (OK)! 4.6.1B

*)

!

! 1 ! ACK negative ! !

! ! FDL/FMA1/2 User Error (UE)! 4.6.1B !

! 2 ! ACK negative ! !

! ! no Resource for Send Data ! !

! ! (& no Response FDL Data) (RR)! - " - !

! 3 ! ACK negative ! !

! ! no Service activated (RS)! - " - !

!4 to 7! Reserved ! !

! 8 ! Response FDL/FMA1/2 Data low ! !

! ! (& Send Data ok) (DL)! 4.6.2B, 4.6.3B !

! 9 ! ACK negative ! !

! ! no Response FDL/FMA1/2 Data, ! !

! ! (& Send Data ok) (NR)! 4.6.1B

*)

!

! 10 ! Response FDL Data high, ! !

! ! (& Send Data ok) (DH)! 4.6.2B, 4.6.3B !

! 11 ! Reserved ! !

! 12 ! Response FDL Data low, ! !

! ! no Resource for Send Data (RDL)! 4.6.2B, 4.6.3B !

! 13 ! Response FDL Data high, ! !

! ! no Resource for Send Data (RDH)! - " - !

! 14,15 ! Reserved ! !

+-------+--------------------------------+----------------------+

! b4 b1 !

! Code No 0 = 0 0 0 0 !

! Code No 15 = 1 1 1 1 !

! !

! ( ) Value of the L/M_status Parameter of the Service !

! Primitives (see Part 3, subclause 4.1.3 and 4.2.3) !

! !

! *) Frames equivalent to Short Acknowledgement SC = E5H, !

! exception: no SC if FDL Status Request !

+---------------------------------------------------------------+

Frame Count Bit

The Frame Count Bit FCB (b6) prevents the duplication of messages at the

responder and the loss at the initiator. However, "Send Data with No Acknowl-

edge" (SDN), "Request FDL Status", "Request Ident" and "Request LSAP Status" are

excluded from this.

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