CD2481 datasheet(97/222 Pages) INTEL

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Programmable Four-Channel Communications Controller — CD2481

Datasheet

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:

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].

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.

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