PROCESSOR TECHNOLOGY CORPORATION
Sol THEORY OF OPERATION SECTION VIII
the light is optically coupled to the base of a photo transistor in
U39 to cause the transistor to conduct. Conduction translates to a
low, or mark, level at the input (pin 13) of U38. Since both the
current loop and RS-232 received data (SLR1/SLR2 and SRD respectively)
share the input to U38, both should not be used simultaneously.
There are five external control signals in the RS-232 section
of the SDI/UART: two are sent to the external device (SRTS and SDTR),
and three are received from the device (SCTS, SCD and SDSR).
SRTS on pin 4 of J1 was discussed earlier. SDTR (serial data
terminal ready) is simply tied to +12 V dc through R24. This indi-
cates to the external device that Sol is connected to it.
SCTS (serial clear to send), SCD (serial carrier detect) and
SDSR (serial data set ready) indicate status of the external device.
They enter Sol on pins 5, 8 and 6 of J1 respectively, and all three
are active high. Following level conversion and inversion in line re-
ceivers U38, data on these lines is gated through noninverting tri-
state buffers U37 to the Internal Data Bus when !PORT_IN_F8 is active.
!PORT_IN_F8 also enables five bits of UART status to be re-
ported over the Internal Data Bus. These are PE, FE, OE, DR and TBRE
on pins 13, 14, 15, 19 and 22 respectively of the UART. They are de-
fined as follows:
PE: Parity Error--received parity does not compare to
that programmed. (Bit INT2)
FE: Framing Error--valid stop bit not received when
expected. (Bit INT3)
OE: Overrun Error--CPU did not accept data before it
was replaced with additional data. (Bit INT4)
DR: Data Ready--data received by UART is available
when requested. (Bit INT6)
TBRE: Transmitter Buffer Register Empty--UART is ready
to accept another word from the Bidirectional
Data Bus. (Bit INT7)
8.5.4 Display Section
An understanding of how characters are formed on the video
monitor will help you follow operation of the display section.
The monitor screen can be thought of as a large matrix of
small light elements, or dots, that can be turned on and off. In
this context the overall video presentation consists of light and
dark dots.
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