Texas Instruments 990 User Manual page 221

Throughput
300 BPS
CPU WI TH HDLC
9600 BPS
PRINTER
...
COMMUNICATIONS
....
...
LINE
PACKAGE
....
LINE
2280617
One solution to this problem is to put some constraint on the
program in the station sending the data. Another possible
solution is to arrange the receiving software at the station to
read tnput messa.ges as rapidly as possible, spool the messages to
a disk file, and interleave write operations to the printer with
read (receive) operations. However, even with this arrangement,
the total number of characters received must not be greater than
the capacity of the disk file to adjust for situations where
interleaving may not be possible (for example, if the printer is
offline).
The DXIO pr imary sta tion communications software 1imi ts the
number of messages that can be sent to or received from any
secondary station. This upper limit, called the window size,
1S
set to a value based on the functionality of the communications
software at the secondary station. In the DXIO HDLe
Communications Package, it is based on the buffer capacity and
software capability at each secondary. For TX5 secondary
stations, the window size is seven and for TM990 or PM550 type
secondary stations, it is one.
Allocation of a window size to each secondary station is not the
ultimate solution, but it does provide some control over the flow
of data. However, in a network where the the number of secondary
stations times the window size is greater than the number of
available buffers,deadlock could still occur. If every task at
every node attempted to send as many messages as the system
allowed, there generally would not be a sufficient number of
buffers to accommodate all messages. Therefore, users should
ensure that each task that communicates does so on an optimal
basis. This approach helps to improve the system throughput and
reduce the possibility of deadlock caused by traffic activity in
excess of the handling ability of the communications package.
B.? TRANSMISSION FORMAT
TRANSMISSIONS UNDER HDLC ARE IN THE FOLLOWING FORMAT:
BYTE 0
2
n-2
n-1
n
ADDRESS
COMMAND OPTIONAL DATA
2280618
Each field is described in the following paragraphs.
2270526-9701
B-9