CD2231 Intel Corporation, CD2231 Datasheet - Page 71
Manufacturer Part Number
CD2231 Intelligent Two-channel Lan And Wan Communications Controller
npad3, 2, 1, 0
Definition of a Valid Frame
This section discusses valid frames from the viewpoint of the CD2231 devices.
All characters are formatted as in the standard async-HDLC format shown in
a channel is placed in the PPP mode, that channel transmits and expects received characters to be as
Async-HDLC and PPP protocols have minimum frame size requirements. However, the CD2231
devices makes no requirement of a minimum frame size.
The frame opens and ends with a flag (7E). The device complies with this in transmit, and requires
opening and closing flags on the receiver. The closing flag from a preceding frame can be the same
flag as the opening flag of the next frame. This is a shared flag. The device can send and receive
both shared or non-shared flags.
The frame never ends with a control-escape followed by a flag (7D–7E). The device does not send
a 7D–7E at the end of a frame in the normal PPP mode. The device can be commanded to send an
abort of either a 7D–7E or a character with a bad stop bit.
If the device receives a frame that ends in a 7D–7E, that frame is indicated to the host as being in
The PPP mode requires transparency as described in
enabled when the channel is in the PPP mode.
Fixed Transmitter Operations
For PPP mode, all transmitted characters are of the format shown in
transmitter always sends an opening flag.
The device transmitter can be control-bit selected for the following options:
When map32 is set, all the characters in the TXACCM (00–1F) are mapped. All 32 characters are
transmitted with a preceding 7D, and with bit 6 flipped.
When map32 is clear, the normal TXACCM is used.
The minimum number idle character times between transmitted frames is programmable from 0–15
If TxGen is set, the device adds the two character FCS at the end of each frame.
If TxGen is clear, the device ends the frame with a closing flag after the last data byte from the host.
When commanded by setting the frame bit in STCR, the device sends one character in the frame with
the Stop bit forced to ‘0’.
Intelligent Two-Channel LAN and WAN Communications Controller — CD2231
6.2.1. There is one exception of an option to transmit a framing error. See
6.2.4. The transparency is always
6.2.1, and the