Omron 8025G Maintenance Manual page 65

THEORY OF OPERATION
SECTION 4
without Processor Card Control.
The CPU sets up a DMA transfer by sending the I/O card a status command and
control information. Once the operation is set
up,
the DMA Processor and I/O cards
assume full control of addressing and other data transfer functions, including the
control of bus priority. The data bus is available for DMA transfers whenever the
Processor Card does not need it (approximately 80% of the time during normal oper-
ation).
In general, DMA operations transfer memory data in blocks. The starting
address and number of words in the block are sent to the DMA processor via a nor-
mal program-processor data transfer. Once this information is in the DMA Processor,
the transfer can begin and is then transferred sequentially, beginning with the start-
ing address. The address is incremented (or decremented) for each transfer until
the number of tra!1sfers defined by the block length is complet,ed. When the transfer
is complete the I/O card becomes ready. That is, it will answer a POLL command and
either be turned off to wait for more data or reset to transfer a new block of data.
4.3 CHARACTER GENERATION
An understanding of how characters are formed on the 8025 CRT screen will
help'in following the card descriptions presented later in this section.
The CRT screen can be thought of as a large matrix of small light elements,
or dots, that can be turned on and off. In this context the overall video present-
ation is made up of light and dark dots.
The.basic display format for the 8025 Terminal is 80 characters per line
with a maximum of 24
line~
per frame (page). Thus, up to 1920 characters can be
displayed per page. Some terminals are configured for a 12 line page format with
a maximum of 960 displayed
chara~ters
per page.
A 10 x 10 dot area on the screen is allotted for each character displayed.
Consequently, each row of characters consisting of eighty 10 x 10 dot areas, requires
ten horizontal scan lines. '
To provide for both character and row spacing, only nine dot lines (L) and
seven dot columns (C) are allotted for character generation. Only seven of the dot
lines, however, are displayable.
In addit\on, a half-dot horizontal shift feature creates an effective 7 x 14
(L x C) dot area for character generation. The function of this shift feature is ex-
plained in Section 4.3.1.
The eighth dot in each scan line is not displayed. It is used as a control
dot, as explained in Section 4.3.2.
4.3.1 Upper Case Characters
5, D, E, F, H, I, K, L, M, N, P, R, T, W, X, and~. In Fi.t',ure
-1-2,
"H" is
used as an example of how characters in this group are formed. As stated earlier,
nine lines are allotted for each character, with only seven lines being displayable.
4-17