Printer Interface Circuit - Tandy 102 Service Manual

Printer Interface Circuit
The printer interface circuit conforms to Centronics standards. As shown in Figure 4-9, the BUSY
signal from the printer is read from the PC2 of the PIO. If the condition is not busy (PC2 = L), the
8-bit parallel data (PAO - PA7) is sent to the printer. Then, by writing data "1 " into bit 1 of the output
port specified by I/O address EOH-EFH, the PSTB signal is sent to the printer.
As soon as the printer receives this PSTB signal, the BUSY signal is changed to H indicating that
the printer is busy. The CPU then waits for a while until this BUSY signal becomes L. The printer
prints the one character corresponding with the 8-bit parallel data. After completion of the one
character printing, the printer sets the BUSY signal to L. Then, the CPU sends the next 8-bit parallel
data. If the printer is in ONLINE condition, the BUSY signal is H and sent to the CPU, passing
through the PC1 of the PIO. However, when in the OFFLINE condition, the BUSY signal is L and
transmission of print data to the printer is inhibited by the CPU.
PSTB
AD1 5 DM14 Q 6
1
STROBE
GND
PAO
PA 1
0
3
A
PDO
GND
PD1
6
GND
10
PA 2
3 PD2
Y6
26 8
8
GND
9
WR PA3
32
P03
0:
9
PA4
PA 5
10
GND
0
12
PD4
GND
U
PD5
W
Z
# GND
z
PA 6
10
PD6 0
16
GND
U
BUSY
PA 7
10
1 PD7
VDD
GND
0:
W
20
GND
i^
PAO-7 21
BUSY
Z_
5- PC2
GND
0
STROBE
R 6433K
_
24
NC
R6833K GND
PCI
BUSY
0.5
A
(Min)
C38 100P
Ill
Figure 4-9. Printer Interface Circuit