Hdlc Mode

The default operating mode used by the SunHSI software is the HDLC full-duplex protocol (mode=fdx). In this mode the transmitter is always enabled. The transmitter sends flag bytes continuously when it is not sending a data frame.

If no message is currently being transmitted, the driver will attempt to start sending its next message. At this point the driver indicates that it is busy transmitting, to prevent the transmission of another message concurrently. The driver also activates a mechanism that ensures that the transmit operation will not hang if the hardware is not responding.

When the transmission is completed, the busy mechanism previously set is cleared and the next message can be transmitted. If the transmission is hung an abort sequence is sent instead of the cyclic redundancy check (CRC), so that the receiver will not interpret the frame as valid data. The message is discarded, and the output error statistic is incremented, which allows for a proper recovery by higher level protocols.

The received data is buffered until a complete frame has been received. If any error occurs during the reception of a frame, the appropriate statistic is incremented and the frame is discarded.

IBM (SDLC) Mode

SDLC mode is designed to support IBM system network architecture (SNA) communications. It uses most of the same protocols used in HDLC mode, with two major exceptions:

■When the line is idle, instead of sending flag bytes the transmitter is disabled.

■The request-to-send (RTS) and clear-to-send (CTS) signals are used to gate transmission.

IBM Full-Duplex Mode

When the SunHSI software is set to this mode (mode=ibm-fdx), the software uses a full-duplex point-to-point communication protocol. Both ends of the link are expected to have RTS and CTS signals asserted at all times when data is being exchanged. When starting a message transmission, the interface raises the RTS signal and expects the CTS signal to be asserted immediately. If this is not done, all messages currently queued for transmission are discarded, and the write operation returns an error.

Chapter 4 SunHSI Utilities 19