
Programmable Four-Channel Communications Controller
—
CD2481
Datasheet
99
7.8
Extended X.21 Mode
Proper selection of options can extend the X.21 mode for some synchronous applications.
Extended X.21 mode does not perform a true programmable sync mode, because the Extended-
X.21 mode keeps passing characters to host though it is in SYN Hunt mode.
7.8.1
Extended X.21 Transmit
Without the ETC (COR2[5]) enabled, the data supplied to the channel is transmitted unaltered.
When no data is available in the transmit FIFO, the ASCII SYN Character (16) is sent to fill idle
time; this is the normal operating mode of X.21. If some other character is required to idle the line,
two methods are available.
First, the idle character can be supplied as normal data; the drawback is that host intervention is
required to either fill the FIFO or supply a new DMA buffer periodically while there is no real data
to send. The second method is to use the ETC to repeat the required data pattern until transmit data
is available. The ETC command functions as shown in
Table 17
(COR2[5] is set).
When an 80 hex character is encountered in the transmit data, it and the following three characters
are treated as a special command; the format of the bytes is as shown in
Table 17
.
To idle-in mark with the RTS* line off, the following command could be written
‘
80 00 FF 00
’
. To
send the character
‘
80
’
while the ETC command is enabled and the RTS* pin is asserted, the
following sequence should be written
‘
80 01 80 01
’
.
7.8.2
Extended X.21 Receive
In Receive mode, the SYN character can be programmed in COR6 to be any required value. When
the channel is initialized, COR6 is initially set to the value
‘
16
’
by the internal code; host software
can reprogram this register to the required SYN value after channel initialization has been
completed. If a parity mode is enabled, the parity bit should not be set in COR6. Synchronization
can be achieved with either a single or double SYN pattern; this is controlled by the SglSyn (single
SYN) bit in COR3.
Junk Data Data
Table 17. Byte Format - ETC Bit Set
Byte 1
Byte 1 must be equal to 80 hex to start a command sequence.
Byte 2
Byte 2 indicates the required state of the RTS* pin.
00 = the RTS* pin is set inactive (high)
01 = the RTS* pin is set active (low)
02
–
FF= reserved
Byte 3
Byte 3 is the required character for transmission. It is sent as an 8-bit character without
parity (any required parity value should be included by the host).
Byte 4
Byte 4 is the number of times the above character should be sent; if set to
‘
0
’
, the
character is sent until new data is supplied
—
but a minimum of three times.