Heathkit H9 Manual page 65

the 8-bit bus and the direction of data flow is control-
Ied by turning the appropriate buffers and latches on
and off at the proper time in the TpU cycle. When data
becomes available from the UART, parallel input, or
keyboard, the I/O circuit board sends an appropriate
data available signal (DAV, at SZ01-3 or DAVo*n at
S701-4) to logic array IC7O2 on the TpU circuit board.
The logic array then checks a number of other inputs
to determine which buffers and latches on the RAM
and counter circuit board, parallel I/O, serial I/O, or
keyboard circuit board will be turned on by TpU
pulse 082 during the write cycle. External devices
must be ready to accept new data, that is, the UART
transmit-buffer must be empty (XBEU, S701-11) and
the parallel output buffer must be empty (XBEP,
s701-1.2).
T h e s pe ci al char ac ter i nputs (p ins 1,2 , B, a nd 1 1 to
logic array lC7O2) are also checked to determine
whether or not the character on the bus is a valid
character. The special character detector is on the I/O
circuit board, and its outputs enter the TPU circuit
board as follows:
Control characters and rubouts are not generally writ-
ten into the RAM unless the plot function is chosen.
However, they are always loaded into the UART and
the parallel output buffers. In the plot mode, all
characters are written into the RAM. In the normal
mode, the only control character that can be written
into the RAM is a carriage retutn, but only if there is a
space at the cursor location. Flip-flop lC7O4B checks
for a space indication before the carriage return is
written.
After all input conditions have settled, TPU cycle 0B2
enables the RAM, parallel, and UART outputs ap-
propriately at pins 13 (ANDed with the 4800 Baud
clock), 14 (load parallel), and 1.7 (re-select RAM) of
lC7O2. See Pictorial 4-1,5 for a summary of how spe-
cial characters are loaded to and from the bus.
Carriage Return
Space
Rubout
Control Character
Line Feed
Back Space
Bell
s701,-7
s 7 0 2 - B
s702-3
s702-2
s 7 0 1 - 9
s 7 0 1 - 8
s703-1,2
CHARACTER
DOES CHARACTER
LOAD INTO _
I
I PARALLEL
RA M lu ARrlo u r
CURSOR
MOVEMENT
ALL CONTROL CHARACTERS - NORMAL - No
Plot - Yes
Yes
Yes Normal - None
Plot - One space to right
* Except for:
CARRIAGE RETURN Normal - Yes
lf space,
Plot - Yes
Yes Yes
Normal - To start of line
Plot - one space to right
L IN E F EED Normal - No
Plot - Yes
Yes
Yes Normal - Down one line
Plot - One space to right
BACK SPACE Normal - No
Plot - Yes
Yes Yes Normal - One space to left
Plot - one space to right
BELL Normal - No
Plot - Yes
Yes
Yes N or m al -N o ne \ B e l l
Plot - One space to right , Rings
RUBOUT
Normal - No
Plot - Yes
Yes Yes
Normal - None
Plot - One space ro right
PICTORIAL 4.1,5