CD2481 datasheet(82/222 Pages) INTEL

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CD2481 — Programmable Four-Channel Communications Controller

Datasheet

7.3.2

Debugging Aids

For debug purposes, the CD2481 can send the sequence ‘ESC’, ‘END’, by setting the AbortTx bit

(STCR[6]). This is intended as an abort frame function. The STCR also has a command for sending

a bad (0 value) Stop bit, which causes a framing error at the receiving end (STCR[2]).

When the CD2481 receives the sequence ‘ESC’, ‘END’, it is reported as ‘receive abort’ in the

RISR register. A bad Stop bit is reported as a FE (framing error) in the RISR.

7.4

MNP4/ARAP Protocol Processing

7.4.1

Framing

An MNP4 (V.42) frame consists of a start flag, data octets, a stop flag, and a 16-bit FCS (frame

check sequence). The FCS uses the polynomial (

), preset to all ‘1’s, transmitted,

and inverted. The character format uses asynchronous framing with 8 data bits, no parity, and one

Stop bit. In-band flow control (XON/XOFF) is not permitted in this mode.

The start flag is a three octet sequence consisting of the start character, escape character, and STX

(0x02). The stop flag is a two octet sequence consisting of the escape character and ETX (0x03).

During transmit, if an escape character is encountered in the data stream, it is duplicated.

Conversely, the receiver discards the second of two sequential escape characters.

MNP4 is the data-link layer of ARAP 1.0 (AppleTalk

 Remote Access Protocol). ARAP 2.0 is the

same as MNP4 except for the two start and escape characters.

The CD2481 uses two Special Character registers (SCHR1 and SCHR2) to hold the definition of

the start and escape characters. There is no mode selection within the CD2481 that allows it to

determine whether it is in an ARAP 1.0 or ARAP 2.0 environment. It builds and detects frames

using the values in the two Special Character registers. The user must load the two Special

Character registers with the appropriate start and escape characters for the version in use during

channel initialization. The two special characters for each protocol are shown in Table 12.

For both versions of ARAP, frames begin with SCHR1, SCHR2, STX, and end with SCHR2 and

ETX:

• ARAP 1.0— SYN, DLE, STX, data, data, data, ... DLE, ETX

• ARAP 2.0— SOH, ESC, STX, data, data, data, ... ESC, ETX

Both versions escape the escape character (in SHCR2) by duplicating it if it appears within the data

stream.

Table 12. Special Character Definition

Special Character

ARAP 1.0

ARAP 2.0

SCHR1 contains the start character

SYN

SOH

SCHR2 contains the escape character

DLE

ESC

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