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Configuring the Terminal
Choosing Buffer Sizes for Multipoint Configurations
When filling in either of the two multipoint configuration menus, two parameters you must
contend with are BufSize and Numbufs.
HumBufs specifies the desired number of datacomm buffers to be allocated for use by the
terminal. Buf
5
i ze specifies the size (in bytes) of each buffer used by the terminal.
The same buffers are used for both transmitting and receiving data. As a result, the terminal
cannot transmit and receive data at the same time. If transmit data is queued, and the
terminal is selected by the host computer, the transmit data will be flushed. If this occurs, the
bell will ring to indicate the data was flushed. If receive data is queued, and the user tries to
enter transmit data, the transmit data will not be accepted until all of the receive data has
been processed.
The memory space allocated to datacomm buffers is obtained from the terminal's display
memory. The more space you allocate for datacomm buffers, the less space you have available
for display.
In the following discussion, procedures for selecting the values for HumBuf sand Buf
S
i ze are
explained.
There is no simple formula for selecting the most appropriate values for HumBuf
5
and BufSi ze.
There are, however, three primary considerations:
1. The overall buffer size (Bu f
5
i ze x HumBu f s) must be large enough to accomodate the largest
block that the host computer will ever send.
2. Each individual buffer (Bu f
5
i z
e)
must be less than or equal to the host computer's receive
buffer size.
3. BufSize x HumBufs must be less than or equal to 4096 bytes for terminal buffers.
If you are merely configuring the terminal's datacomm buffers to match those of a host
computer, then the above guidelines will suffice. However, if you are responsible for configur-
ing datacomm buffers both at the terminal and at the host computer, the situation becomes
more complicated. The following paragraphs present some of the things you should consider.
Each time the terminal operator presses _, a block of data is transferred from the cursor
active workspace to the datacomm output buffer(s) for the associated port. If this block of data
is larger than a single buffer, then it will fill as many buffers as necessary. When the data is
transmitted to the host computer, each buffer is transmitted as a block. If the data block
required more than one buffer, it will require a multiple block transmission (with an <ETB> at
the end of each except the final one, which is terminated by an <ETX
».
Such a multiple block
transmission requires considerably more line control activity (and physical line turnarounds
in a half duplex configuration) than if the data were transmitted as a single block.
Therefore, you will want to consider the amount of data to be transmitted in response to a
typical _
key usage and tailor your buffer sizes accordingly.
3-26
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