User`s guide

reported to the application, and checksum errors detected by the SLTA-10 Adapter
will cause the message to be ignored.
This protocol is used when the host is capable of accepting asynchronously occurring
input data without losing characters. The host is also relieved of the obligation of
responding to an ALERT character within 50 ms. This protocol may therefore be
used by an application-level handler calling an interrupt-driven buffered serial device
driver. Drivers with these characteristics are typically provided with real time
operating systems such as VRTX or time-sharing operating systems such as UNIX or
VMS. In this case, these drivers should be set up for binary data communications
without software flow control.
The buffered link protocol should not be used when the SLTA-10 Adapter is attached
to a modem.
The buffered link protocol can only be used on multitasking operating systems such
as UNIX if the host application executes often enough to empty any incoming buffers.
For example, if the SLTA-10 Adapter is receiving 70 packets per second, and each
packet is 25 bytes, the host will receive 1750 bytes per second. If the host has a serial
input buffer of 256 bytes, the buffer will fill within 150 milliseconds if the host
application is preempted. If the host application is preempted for longer than 150
milliseconds, incoming data will be lost due to lack of serial buffer space.
In this case,
the ALERTlACK protocol should be used, or the buffer space increased to handle the
worst case traffic during the maximum preemption period.
Transport Layer Protocol
When used with a local host, the SLTA-10 Adapter assumes a reliable connection and
does not use a transport layer protocol. When used with a remote host, the SLTA-10
Adapter assumes that the link may not be reliable and enables the reliable transport
protocol. The reliable transport protocol adds an ACWNACK transport protocol to
the network interface protocol. A sequence number is also added to the link-layer
header. This protocol can therefore recover from checksum errors on the host to
SLTA-10 Adapter link.
The reliable transport protocol is enabled on the SLTA-10 Adapter with the Remote
Host option selected by the Switch2/CFGB input as described in Chapter 4. The
reliable transport protocol is enabled on the DOS network driver with the /M option
as described in Chapter 8. The reliable transport protocol is not supported by the
UNIX network driver.
The link-layer header contains an ALERT (0x01) byte, a sequence number, and a
length byte followed by a one’s complement of the length byte.
These values are
always validated by the receiver before accepting the rest of the message.
Following
the length bytes is the network interface command.
See Appendix D of the Host
Application Programmer’s Guide for a description of the command byte structure. If
the message contains a data field it follows the command byte. Finally, a checksum
terminates the sequence.
The ALERT/ACK link protocol should be used with remote hosts.
With this protocol,
the sender will start the sequence by transmitting the ALERT byte. When this byte
is received by the receiver, that device responds by transmitting the ALERT ACK
9-8 Creating an SLTA-10 MIP Mode Driver