CD2481 Datasheet, PDF (79/222 Pages) Intel Corporation – Programmable Four-Channel Communications Controller

Programmable Four-Channel Communications Controller — CD2481
7.2.4.4
7.2.5
7.2.6
7.2.6.1
7.2.6.2
Mapped Characters in the FCS Field
Whenever the transmitter sees that the FCS result to be transmitted contains a mapped character,
the transmitter handles that character as any other mapped character. See Section 5.3.
For example, suppose that an FCS field just happened to be A7–7E. In that case, the transmitter
would send three characters in the FCS field: A7–7D–5E. The receiver would convert the received
FCS back to A7–7E before completing the FCS computation.
Definition of a Valid Frame
Valid frames, from the viewpoint of the CD24XX chips, are:
All characters are formatted as in the standard async-HDLC format shown in Section 7.2.1. When
a channel is placed in the PPP mode, that channel transmits and expects received characters to be as
shown in Section 7.2.1. There is one exception of an option to transmit a framing error. See Section
7.2.6.2 and Section 7.2.6.4.
Async-HDLC and PPP protocols have requirements of minimum frame size. However, the
CD24XX chips makes no requirement of a minimum frame size.
The frame opens and ends with a flag (7E). The chip complies with this in transmit, and requires
opening and closing flags on the receiver. The closing flag from a preceding frame may be the
same flag as the opening flag of the next frame. This is a shared flag. The chip can send and receive
shared or non-shared flags.
The frame never ends with a control-escape followed by a flag (7D–7E). The chip does not send a
7D–7E at the end of a frame in the normal PPP mode. The chip may be commanded to send an
abort of either a 7D–7E or a character with a bad stop bit.
If the chip receives a frame that ends in a 7D–7E, that frame is indicated to the host as being in
error.
PPP requires transparency as described in Section 7.2.4. When the channel is in the PPP mode,
transparency is always enabled.
Transmitter
Fixed Transmitter Operations
For PPP mode, all transmitted characters are of the format shown in Section 7.2.1, and the
transmitter always sends an opening flag.
Transmitter Options
The device transmitter may be control-bit selected for options provided in the following table:
Datasheet
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