Flat screen printing machine signal communication circuit

The printing unit and central control unit of the flat screen printing machine adopt the RS-323C standard, and the printing unit sends information to the host computer. The circuit is shown in the figure
. Data is sent from pin @ of the chip 8251A. If a high level , VT4, VT5, and VT6 are turned on, and the printing unit
sends ; if a low level signal is sent out, VT4, VT5, VT6 is turned off, VT2 and VT3 are turned on, and the printing unit
sends a +10V level signal to the host.
    When the printing unit is reset or does not send a signal, VT4 is turned on. At this time, each transistor is turned off, and the printing unit is in a high-impedance state relative to the host. When the printing
unit receives information from the host, the signal is received by RXD, current-limited by resistor R26 (lOk), sent to the Schmidt NAND gate 74LS132
for reverse direction, and then reaches pin ③ of the 8251 for reading. This completes communication with the central control unit.