C-22 Sun Ultra 60 Service Manual • August 2001
C.1.11 Serial Port
The system unit incorporates two serial ports. Each serial port is synchronous and
asynchronous with full modem controls.All serial port functions are controlled by a
serial port controller that is electrically connected to the system through the EBus.
Line drivers and line receivers control the serial port signal levels and provide
RS-232 and RS-423 compatibility.Each serial port interfaces through its own DB-25
connector.
The major features of each serial port include:
■Twofully functional synchronous and asynchronous serial ports
■DB-25 connectors
■Increased baud rate speed (to 384 Kbaud, synchronous, 460.8 Kbaud,
asynchronous)
■Variableedge rate for greater performance
■EBus interface
FIGUREC-7 shows a functional block diagram of the serial port.

Serial Port Components

Serial port components include a serial port controller,line drivers, and the line
receivers.
The serial port controllercontains 64-byte buffers on both the input and output. This
enablesthe serial port to require less CPU bandwidth. Interrupts are generated when
the buffer reaches 32 bytes or half full. The serial port controllercontains its own
crystal oscillator that supports rates of up to 921.6 Kbaud.
The line drivers and line receivers arecompatible with both RS-232 and RS-423. Two
system board jumpers are used to set the line drivers and line receiversto either
RS-232 orRS-423 protocols. The line driver slew rate is also programmable. For baud
rates over 100K, the slew rate is set to 10 Vdc/µsec. For baud rates under 100K, the
slew rate is set to 5 Vdc/µsec.