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ISAC-SX
PEB 3086
Description of Functional Blocks
Data Sheet
146
2003-01-30
3.8.4
Data Transmission
3.8.4.1
The cyclic transmit FIFO buffers with a length of 64 byte for D-channel and 128 byte for
B-channel have FIFO block sizes (thresholds) of
16 or 32 bytes (programmable) for D-channel and
32 bytes for B-channels
which can be selected by setting the corresponding XFBS bits in the EXMx registers.
There are three different interrupt indications in the ISTAx registers concerned with the
transmission of data:
–
XPR
(
T
ransmit
P
ool
R
eady) interrupt, indicating that a data block of up to 16 or 32 byte
(D-channel), or 32 byte (B-channel) can be written to the XFIFOx (fixed block size).
An XPR interrupt is generated either
after an XRES (Transmitter Reset) command (which is issued for example for frame
abort) or
when a data block from the XFIFOx is transmitted and the corresponding FIFO
space is released to accept further data from the host.
–
XDU
(
T
ransmit
D
ata
U
nderrun) interrupt, indicating that the transmission of the
current frame has been aborted (seven consecutive ’1’s are transmitted) as the
XFIFOx holds no further transmit data. This occurs if the host fails to respond to an
XPR interrupt quickly enough.
– Only valid for D-channel:
XMR
(
T
ransmit
M
essage
R
epeat) interrupt, indicating that the transmission of the
complete last frame has to be repeated as a collision on the S bus has been detected
and the XFIFOx does not hold the first data bytes of the frame (collision after the 16th/
32nd byte or after the 32nd byte of the frame, respectively).
Structure and Control of the Transmit FIFO
The occurence of an XDU or XMR interrupt clears the XFIFOx and an XMR interrupt
is issued together with an XDU or XMR interrupt, respectively. Data cannot be written
to the XFIFOx as long as an XDU/XMR interrupt is pending.
Three different control commands are used for transmission of data:
–
XTF
(
T
ransmit
T
ransparent
F
rame) command, telling the ISAC-SX that up to 16 or 32
byte (D-channel) or 32 (B-channel) have been written to the XFIFOx and should be
transmitted. A start flag is generated automatically.
–
XME
(
T
ransmit
M
essage
E
nd) command, telling the ISAC-SX that the last data block
written to the XFIFOx completes the corresponding frame and should be transmitted.
This implies that according to the selected mode a frame end (CRC + closing flag) is
generated and appended to the frame.
–
XRES
(
T
ransmitter
R
eset) command, resetting the HDLC transmitter and clearing the
transmit FIFO of any data. After an XRES command the transmitter always sends an
abort sequence, i.e. this command can be used to abort a transmission. Pending