Character Level Flow Control - Eads Astrium SMCS332SpW User Manual

Character Type
Data character
control characters:
Flow control
Normal End of Packet
Error End of Packet
Escape
control codes:
Null
Time code
P = Parity bit
D = Data bit
3.2

Character level flow control

Character level flow control is performed in each SpaceWire module, and the additional flow control characters used are not
visible to the higher-level packet protocol. The character level flow control mechanism prevents a sender from overrunning
the input buffer of a receiving link. Each receiving link input contains a buffer for at least 8 normal-characters (16 normal-
characters of buffering is in fact provided). Normal-characters are data character and EOP/EEP. Whenever the link input has
sufficient buffering available to consume a further 8 normal-characters a FCT is transmitted on the associated link output,
and this FCT gives the sender permission to transmit of further 8 normal-characters. Once the sender has transmitted 8
normal-characters it waits until it receives another FCT before transmitting any more tokens. The provision of more than 8
normal-characters of buffering on each link input ensures that in practice the next FCT is received before the previous block
of 8 normal-characters has been fully transmitted, so the character level flow control does not restrict the maximum
bandwidth of the link.
For further information see [AD1]
SMCS332SpW
User Manual
Abbreviation
FCT
EOP
EEP
ESC
NULL
– All Rights Reserved – Copyright per DIN 34 –
EADS Astrium GmbH, ASE2
Doc No: SMCS_ASTD_UM_100
Issue: 1.4
Updated: 9-Sep-2006
Page: 15 of 131
Coding
P0DDDDDDDD
P100
P101
P110
P111
ESC + FCT
P1110100
ESC + DATA
P11110DDDDDDDD