Epson LQ-1500 Service Manual page 76

3.2.3 Operating principle
3.2.3.1 Data entry
Serial data entry is performed under the preset conditions.
(1) The serial data transferred from the host computer is input to CN3 pin 3 (RXD) or pin 25 (TTY RXD).
(2) The data input to pin 3 (RXD) or pin 25 (TTY RXD) is entered in the CPU. Then, the data is converted
from serial to parallel inside the CPU.
(3) The converted parallel data is stored in the 2 K-byte input buffer.
Concerning this stored data, its status is made known to the host computer (flag control or X-ON/X-
OFF control) after performing buffer full/empty control.
(4) The stored data is transferred to the print buffer via 8255AC-5 (peripheral interface) as required.
3.2.3.2 Buffer full/empty control
(1) Flag control
The status flag is used for handshaking with the host computer. (When a status flag is emitted, data
entry to the 2 K-byte buffer is possible.)
a) Using RS232C
When the rest of the 2 K-byte input buffer become 16 bytes, interface connector CN3 pin 11 (REV)
and pin 20 (DTR) go mark state, telling the host computer that the buffer is full.
Data is processed after it has been. When the set value of the buffer recovery timing corresponds
with the bytes available in the buffer, pins 11 and 20 go LOW, signaling the host computer that
data entry is possible.
b) Using current loop
The operating principle is the same as for RS-232C. However, when the 2 K-byte buffer is full, the
impedance between interface connector CN3 pins 17 (TTY TXD) and 24 (TTY TXD R) is made HIGH
to tell the host computer that the buffer is full. When the set value of the buffer recovery timing (DIP
3,3-4) corresponds with space available in the buffer, impedance between pins 17 and 24 is made
LOW, telling the host computer that data entry is possible.
(2) X-ON/X-OFF control
In the X-ON/X-OFF control, when data entry is possible, DC1 (X-ON) is transmitted to the host com-
puter, while when impossible, DC3 (X-OFF) is transmitted.
a) Using RS-232C
X-ON (DC1 : 11
(TXD) until the remaining space for the serial data stored in the 2 K-byte buffer via RXD is for 16
bytes. Thus, the host computer is informed that data entry is possible.
When the remaining space for the data is 16 bytes, X-OFF (DC3:
Thus, the host computer is informed that the buffer is full.
b) Using current loop
X-ON is transmitted from interface connector pins 17 (TTY TXD) and 24 (TTY TXD R) every 8 bytes
until the rest space for the serial data stored in the 2 K-byte buffer through TTY RXD and TTY RXD R
is for 16 bytes. When it is for 16 bytes, X-OFF (DC3:
TXD R.
is periodically (every 8 bytes) transmitted from interface connector CN3 pin 2
H)
is transmitted from TTY TXD and TTY
13H)
3 - 1 5
is transmitted from TXD.
13H)