
Communications Processor (CP)
MOTOROLA
MC68302 USER’S MANUAL
4-107
NOTE
This error can occur only on synchronous links.
2. Clear-To-Send Lost (Collision) During Message Transmission. When this error occurs 
and the channel is not programmed to control this line with software, the channel ter-
minates buffer transmission, closes the buffer, sets the CTS lost (CT) bit in the BD, 
and generates the transmit error (TXE) interrupt (if enabled). The channel resumes 
transmission after the reception of the RESTART TRANSMIT command.
Reception Errors:
1. Carrier Detect Lost During Message Reception. When this error occurs and the chan-
nel is not programmed to control this line with software, the channel terminates mes-
sage reception, closes the buffer, sets the carrier detect lost (CD) bit in the BD, and 
generates the receive block (RBK) interrupt (if enabled). This error has the highest pri-
ority. The rest of the message is lost, and other errors in that message are not 
checked.
The channel will enter hunt mode immediately. It is possible that a SYN1–SYN2- 
(SOH,DLE,ENQ) sequence in data will be incorrectly interpreted as the start of the 
next header, but this "header" will have a CRC error.
2. Overrun Error. The DDCMP controller maintains an internal three-byte FIFO for re-
ceiving data. The CP begins programming the SDMA channel (if the data buffer is in 
external memory) and updating the CRC when the first word is received into the FIFO. 
If the receive FIFO overrun error occurs, the channel writes the received data byte to 
the internal FIFO on top of the previously received byte. The previous data byte is lost. 
Then the channel closes the buffer, sets the overrun (OV) bit in the BD, and generates 
the receive block (RBK) interrupt (if enabled).
The channel will enter hunt mode immediately. It is possible that a SYN1–SYN2- 
(SOH,DLE,ENQ) sequence in data will be incorrectly interpreted as the start of the 
next header, but this “header” will have a CRC error.
3. CRC1 (Header CRC) Error. When this error occurs, the channel writes the received 
CRC to the data buffer, closes the buffer, sets the CRC error (CR) bit in the BD, gen-
erates the RBK interrupt (if enabled), increments the error counter (CRC1EC), and en-
ters hunt mode.
When this error occurs on data-and maintenance-message header fields, the channel 
will enter hunt mode immediately. It is possible that a SYN1–SYN2-(SOH,DLE,ENQ) 
sequence in data will be incorrectly interpreted as the start of the next header, but this 
“header” will have a CRC error.
4. CRC2 (Data or Maintenance CRC) or CRC3 (Control Message) Error. When this error 
occurs, the channel writes the received CRC to the data buffer, closes the buffer, sets 
the CRC error (CR) bit in the BD, and generates the RBK interrupt (if enabled). The 
channel also increments the CRC2EC counter and enters hunt mode.
5. Framing Error. A framing error is detected by the DDCMP controller when no stop bit 
is detected in a received data string. When this error occurs, the channel writes the 
received character to the buffer, closes the buffer, sets the framing error (FR) bit in the 
BD, and generates the RBK interrupt (if enabled). When this error occurs, parity is not 
checked for this character.