Epson PX-8 Technical Manual page 59

REV.-A
Handshaking between the
l80
and 6303 is performed via the two status registers in SED1320
which are respectively assigned to the CPUs. Each of the registers has Input Buffer Full (IBF), Out-
put Buffer Full (IBF). and Flag (FI) bits as shown in Fig. 2-31. Data outputs (write) and inputs (read)
are controlled by the state of these status bits. The handshaking between the two CPUs is ex-
plained below.
zao
SED 1320
6303
x
X :>
Status registers
F1
fE-
~
F1
------- -------
OOOEH: Read
OOOEH .
OOOFH: Wnte
~
-------- 1---------
S
R
IBF
-
-
-
- ---
IBF
R
-
---:--<:--
S
---------
1---------
S
-
--
-
-
- -
-
R
OBF
--
OBF
cf!-Q
Fig. 2-31 Handshaking Between
zao
and 6303
The status registers are accessed from both
l80
and are 6303 and used as follows:
Output
When either the
l80
or 6303 outputs 1-byte data, the OBF bit of one status register and the IBF
bit of the other status register are set. This informs the other CPU that the 1-byte data has been
stored in the gate array SED1320, ready to be read.
Input
When the CPU reads the data from SED1320, the IBF bit of its status register and the OBF bit of
the other register are reset after the data is read in. These input and output operations are
asynchronous.
2-35