nslink − Device driver for DeviceMaster
Kernel module
The nslink device driver provides support for Comtrol DeviceMaster network attached serial ports. It provides standard serial port semantics via devices /dev/ttySI[0-n].
The NS-Link driver implements a low-level serial device that runs underneath the same tty line discipline layer as do other serial devices. That means that usage for a DeviceMaster is almost identical to that of any other serial port. It is possible due to network or other hardware problems that the host my lose contact with the low-level serial device (something that does not usually happen with internal serial boards). This means that the NS-Link driver may return errors not typically seen when using other serial drivers. The table below shows the possible errors:
Reconnect
operation !CLOCAL && !CD Link Down Link Timeout
Refusal[3]
--------- -------------- ----------- ------------
---------
read() EOF block[1] EIO EIO
write() EIO success/block[2] EIO ENOTCONN
open() block till CD EBUSY EBUSY EBUSY
|
[1] |
Assuming that the fd is in blocking mode and there is no data in the line discipline rx buffer, a read() will block until either the connection returns and data is available, or until the link times out. | ||
|
[2] |
Up to 4K of write data will be queued, and subsequent writes will block until the connection resumes or the link times out. | ||
|
[3] |
A reconnect refusal occurs when the network link between the host and the DeviceMaster times out and another host opens the port. If the connection to the original host returns and the application still has the device open, the original host attempts to reconnect to the port and be refused by the DeviceMaster since the port is in-use. |
Communications with the DeviceMaster may use either TCP/IP or the raw packet Ethernet interface.
The default behavior of the NS-Link driver is to consolidate network traffic bound towards the DeviceMasters and send data to all DeviceMasters once every 10ms. This can introduce a delay of up to 10ms when data is written to a DeviceMaster.
Setting the low_latency flag on a DeviceMaster causes data written to that device to be sent immediately. This reduces the latency in outbound data at the expense of increased CPU and network usage.
The low_latency flag may be set in the nslink.conf(5) file, by using the setseral(8) utility (see below) or with the underlying TIOCSSERIAL and TIOCGSERIAL ioctl() calls.
By default,
serial data received by the DeviceMaster is sent to the
NS-Link driver once every 10ms. This can introduce a delay
of up to 10ms in the recieve data path. In the DeviceMaster,
this latency can be reduced in two ways:
Inter-character timeout
It is possible to configure an inter-character timeout that causes the DeviceMaster to send queued receive data when a gap is detected in the receive serial data stream. An inter-character timeout may be configured either in the nslink.conf(5)file, by using the setseral(8) utility (see below) or with the underlying TIOCSSERIAL and TIOCGSERIAL ioctl() calls.
Scan period
It is also possible to change the DeviceMaster’s scan period from the default 10ms to a lower value. This also decreases latency, but increases CPU and network usage. Configuring an inter-character timeout is generally a better way to reduce latency in receive data. The scan period is configured in the nslink.conf(5) file. A value of 0 causes the DeviceMaster to use its default scan period (10ms).
Retransmit Timer
It is also possible to change the DeviceMaster’s retransmit timer from the default 200ms to a lower value. This decreases the wait before retransmitting a packet, but increases CPU and network usage. The timer is configurable for 40ms to the default value of 200ms, in 20ms intervals. The retransmit timer is configured in the nslink.conf(5) file. A value of 0 causes the DeviceMaster to use its default timer value (200ms).
The standard
setserial(8) or the underlying TIOCSSERIAL and
TIOCGSERIAL ioctl() calls may be used to access the
following attributes of a DeviceMaster:
low_latency flag (read-write)
When set, causes write() data to be written immediately to the DeviceMaster.
irq (read-write)
The inter-character timeout value (in milliseconds) for the port. A value of 0 disables the inter-character timeout feature.
port (read-only)
Contains either the IP address or the low-order 32 bits of the MAC address of the DeviceMaster.
line (read-only)
Contains the port number within the DeviceMaster (e.g 0-7 for a port on an 8-port DeviceMaster).
Configuration of the NS-Link driver is done via the configuration file /etc/nslink.conf(5) and via setserial(8) as described above.
None
/proc/driver/nslink/status
If the Linux kernel supports the /proc filesystem, this file contains the current status of the DeviceMaster the driver is configured to control.
/proc/driver/nslink/ports
If the Linux kernel supports the /proc filesystem, this file contains the current status of the individual ports for which the driver is configured.
devmast.bin
Application binary file downloaded to DeviceMaster by the load operation. Search path: current directory, /etc.
nslinkd(8), nslinkadmin(8), nslinkrelease(8), nslinktool(8),setserial(8)
Check the release notes file for an up-to-date list.
ftp://ftp.comtrol.com/