CD2481 Datasheet, PDF (97/222 Pages) Intel Corporation – Programmable Four-Channel Communications Controller

Programmable Four-Channel Communications Controller — CD2481
NOTES:
1. The syn hunt state not shown. The syn hunt state consists of two sub-states that look for a
received SYN-SYN pattern. Upon receiving SYN-SYN, the state moves to BGETHDR.
2. Conditions for the state table:
— Bisync
— Receive
7.7
X.21 Call Set-Up Mode
The X.21 call setup protocol uses a combination of synchronous data and control leads to control
call progress, between a DCE and a DTE. The data can be in the form of steady state conditions (all
‘1’s, all ‘0’s, alternating ‘1’s and ‘0’s) or character synchronous data. The control leads are used in
conjunction with the steady state data conditions to pass change of state information between the
DCE and DTE. To enable detection of the steady state conditions, the SSDE bit (COR3[4]) must be
set.
7.7.1
X.21 Transmit
To minimize the CPU intervention in the transmit direction, a modified version of the ETC
(embedded transmit command) is used. ETC mode is controlled by COR2. When enabled, ETC
mode provides a means of transmitting steady state, or repetitive data patterns synchronized to the
control lead. The ETC consists of a sequence of four bytes passed to the CD2481 as normal
transmit data, with the following format:
Table 16. ETC Byte Sequence
Byte 1 This byte must be 80 hex to indicate the start of a command sequence.
Byte 2
This byte indicates the required state of the control lead
00 = sets the control lead off
01 = sets the control lead on
02-FF = reserved.
Byte 3
This byte the is data character to be transmitted, and is transmitted as is, with no parity added. If
parity is required, it must be included in the byte (that is, to transmit ‘+’ with odd parity, the value AB
would be loaded).
Byte 4
This byte is a count of the number of times to transmit the character. If the count is ‘0’, the character
are sent indefinitely until new data is made available (this would be the normal mode for the steady
state conditions). In this case, when the count is ‘0’, the CD2481 always sends the data a minimum
of three times even if new data is made available sooner.
7.7.2
X.21 Receive
In receive, the CD2481 validates the steady state conditions, passing just the change of state
information to the CPU. Steady state conditions validated are:
• All ‘1’s
• All ‘0’s
• Alternating ‘1’s and ‘0’s
• SYN the characters programmed in SCHR[1-3].
Datasheet
97