Packet structure

+---------------------------+
|            Flags          |
|          (1 Octet)        |
+---------------------------+
|           Signals         |
|          (1 Octet)        |
+---------------------------+
|          Errors 1         |
|          (1 Octet)        |
+---------------------------+
|          Errors 2         |
|          (1 Octet)        |
+---------------------------+
|          Protocol         |
|          (1 Octet)        |
+---------------------------+
|           Payload         |
.                           .
.                           .
.                           .

Description

The bits in the Flags field are:

• 0x01 - set if the frame was transmitted from the remote to the IOP, clear if the frame was received by the remote from the IOP;
• 0x80 - set if there was a buffer failure.

The Signals field shows the state of various RS-232 signals:

• 0x01 - Data Set Ready;
• 0x02 - Data Terminal Ready;
• 0x04 - Clear To Send;
• 0x08 - Request To Send;
• 0x10 - Data Carrier Detect.

The bits in the Errors 1 field are, for frames received by the remote from the IOP:

• 0x01 - framing error;
• 0x02 - parity error;
• 0x04 - collision;
• 0x08 - long frame received;
• 0x10 - short frame.

The Errors 1 field is unused for frames transmitted from the remote to the IOP.

The bits in the Errors 2 field are, for frames received by the remote from the IOP:

• 0x01 - non-octet-aligned frame;
• 0x02 - abort received;
• 0x04 - carrier lost;
• 0x08 - DPLL error;
• 0x10 - overrun error;
• 0x20 - frame length violation;
• 0x40 - CRC error;
• 0x80 - break received.

and, for frames transmitted from the remote to the IOP:

• 0x01 - transmitter underrun;
• 0x02 - Clear To Send lost;
• 0x04 - UART error;
• 0x08 - retransmit limit reached.

The Protocol field indicates the protocol; the values of that field are:

• 1 - LAPB;
• 2 - Ethernet;
• 3 - asynchronous (interrupt I/O);
• 4 - asynchronous (block I/O);
• 4 - asynchronous (block I/O);
• 5 - IPARS/ALC (International Passenger Airline Reservation System/Airline Link Control);
• 6 - UTS;
• 7 - PPP/HDLC;
• 8 - SDLC;
• 9 - Token Ring;
• 16 - I2C;
• 17 - DPM link;
• 18 - Frame Relay.