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man pages section 7: Device and Network Interfaces

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Updated: July 2017



zsh - On-board serial HDLC/SDLC interface


#include <fcntl.h>
open(/dev/zshn, mode );
open(/dev/zsh, mode );


The zsh module is a loadable STREAMS driver that implements the sending and receiving of data packets as HDLC frames over synchronous serial lines. The module is not a standalone driver, but instead depends upon the zs module for the hardware support required by all on-board serial devices. When loaded this module acts as an extension to the zs driver, providing access to an HDLC interface through character-special devices.

The zshn devices provide what is known as a data path which supports the transfer of data via read(2) and write(2) system calls, as well as ioctl(2) calls. Data path opens are exclusive in order to protect against injection or diversion of data by another process.

The zsh device provides a separate control path for use by programs that need to configure or monitor a connection independent of any exclusive access restrictions imposed by data path opens. Up to three control paths may be active on a particular serial channel at any one time. Control path accesses are restricted to ioctl(2) calls only; no data transfer is possible.

When used in synchronous modes, the Z8530 SCC supports several options for clock sourcing and data encoding. Both the transmit and receive clock sources can be set to be the external Transmit Clock (TRxC), external Receive Clock (RTxC), the internal Baud Rate Generator (BRG), or the output of the SCC's Digital Phase- Lock Loop (DPLL).

The Baud Rate Generator is a programmable divisor that derives a clock frequency from the PCLK input signal to the SCC. A programmed baud rate is translated into a 16-bit time constant that is stored in the SCC. When using the BRG as a clock source the driver may answer a query of its current speed with a value different from the one specified. This is because baud rates translate into time constants in discrete steps, and reverse translation shows the change. If an exact baud rate is required that cannot be obtained with the BRG, an external clock source must be selected.

Use of the DPLL option requires the selection of NRZI data encoding and the setting of a non-zero value for the baud rate, because the DPLL uses the BRG as its reference clock source.

A local loopback mode is available, primarily for use by the syncloop(1M) utility for testing purposes, and should not be confused with SDLC loop mode, which is not supported on this interface. Also, an auto-echo feature may be selected that causes all incoming data to be routed to the transmit data line, allowing the port to act as the remote end of a digital loop. Neither of these options should be selected casually, or left in use when not needed.

The zsh driver keeps running totals of various hardware generated events for each channel. These include numbers of packets and characters sent and received, abort conditions detected by the receiver, receive CRC errors, transmit underruns, receive overruns, input errors and output errors, and message block allocation failures. Input errors are logged whenever an incoming message must be discarded, such as when an abort or CRC error is detected, a receive overrun occurs, or when no message block is available to store incoming data. Output errors are logged when the data must be discarded due to underruns, CTS drops during transmission, CTS timeouts, or excessive watchdog timeouts caused by a cable break.


The zsh driver supports several ioctl() commands, including:


Return a struct scc_mode containing parameters currently in use. These include the transmit and receive clock sources, boolean loopback and NRZI mode flags and the integer baud rate.


The argument is a struct scc_mode from which the SCC channel will be programmed.


Return a struct sl_stats containing the current totals of hardware-generated events. These include numbers of packets and characters sent and received by the driver, aborts and CRC errors detected, transmit underruns, and receive overruns.


Clear the hardware statistics for this channel.


Returns the currently set baud rate as an integer. This may not reflect the actual data transfer rate if external clocks are used.


Returns the current state of the CTS and DCD incoming modem interface signals as an integer.

The following structures are used with zsh ioctl() commands:

struct  scc_mode {
	char  sm_txclock;   /* transmit clock sources */
	char  sm_rxclock;   /* receive clock sources */
	char  sm_iflags;    /* data and clock inversion flags (non-zsh) */
	uchar_t  sm_config; /* boolean configuration options */
	int  sm_baudrate;   /* real baud rate */
	int  sm_retval;     /* reason codes for ioctl failures */
struct  sl_stats {
	long  ipack;        /* input packets */
	long  opack;        /* output packets */
	long  ichar;        /* input bytes */
	long  ochar;        /* output bytes */
	long  abort;        /* abort received */
	long  crc;          /* CRC error */
	long  cts;          /* CTS timeouts */
	long  dcd;          /* Carrier drops */
	long  overrun;      /* receive overrun */
	long  underrun;     /* transmit underrun */
	long  ierror;       /* input error */
	long  oerror;       /* output error */
	long  nobuffers;    /* receive side memory allocation failure */


An open() will fail if a STREAMS message block cannot be allocated, or:


The unit being opened does not exist.


The device is in use by another serial protocol.

An ioctl() will fail if:


An attempt was made to select an invalid clocking source.


The baud rate specified for use with the baud rate generator would translate to a null time constant in the SCC's registers.



character-special devices


header file specifying synchronous serial communication definitions


See attributes(5) for descriptions of the following attributes:


See Also

syncinit(1M), syncloop(1M), syncstat(1M), ioctl(2), open(2), read(2), write(2), attributes(5), zs(7D)

Refer to the Zilog Z8530 SCC Serial Communications Controller Technical Manual for details of the SCC's operation and capabilities.


zsh data open failed, no memory, rq=nnn

zsh clone open failed, no memory, rq=nnn

A kernel memory allocation failed for one of the private data structures. The value of nnn is the address of the read queue passed to open(2).

zsh_open: can't alloc message block

The open could not proceed because an initial STREAMS message block could not be made available for incoming data.

zsh: clone device dmust be attached before use!

An operation was attempted through a control path before that path had been attached to a particular serial channel.

zshn: invalid operation for clone dev.

An inappropriate STREAMS message type was passed through a control path. Only M_IOCTL and M_PROTO message types are permitted.

zshn: not initialized, can't send message

An M_DATA message was passed to the driver for a channel that had not been programmed at least once since the driver was loaded. The SCC's registers were in an unknown state. The S_IOCSETMODE ioctl command performs the programming operation.

zshn: transmit hung

The transmitter was not successfully restarted after the watchdog timer expired.