User`s guide

device responds by transmitting the ALERT ACK byte (value FE hex).
This low level
handshaking process prevents the sender from transmitting the rest of the sequence
before the receiving device is ready.
Once the ALERT ACK byte is received by the
sender it sends the rest of the message without any other interactions.
Sender
Receiver
I
1 ALERT ACK (FE) 1
I
L
I
I
[ data ]
I
1 checksum 1
Figure 9.2 SLTA-10 Adapter ALERT/ACK Link Protocol
The link-layer header contains 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 length byte value describes the length of the network interface command byte
plus the length of the data field. This value will always be at least 1. The checksum
is a two’s complement of the sum of the command byte and all of the bytes in the data
field, if it exists. Checksum errors detected by the host will cause an error to be
reported to the application, and checksum errors detected by the SLTA-10 Adapter
will cause the message to be ignored.
The SLTA-10 Adapter places the following requirements on the rate of the received
serial data stream. When receiving, the maximum wait period for the length byte
following the transmission of the ALERT ACK byte is 1OOms (or 1 second when
attached to a modem). All subsequent bytes received must occur within 1OOms after
the previous byte, otherwise the SLTA-10 Adapter receive process will abort.
Likewise, the SLTA-10 Adapter uses a wait period of 1OOms (or 1 second when
attached to a modem) before aborting for the reception of the ALERT ACK when
9-6 Creating an SLTA-10 MIP Mode Driver