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Electrical Characteristics
Drivers operate at a nominal 5-VDC transistor-transistor logic (TTL) levels. The
maximum open circuit voltage is 5.5 VDC and the minimum is -0.5 VDC. A logic
high-level signal is at least 2.4 VDC at a source current of 0.32 mA and a logic
low-level signal is no more than 0.4 VDC at a sink current of 14 mA.
Receivers also operate at nominal 5-VDC TTL levels and can withstand peak voltage
transients between -2 VDC and 7 VDC without damage or improper operation. The
high-level threshold is less than or equal to 2.0 VDC and the low-level threshold is at
least 0.8 VDC. Sink current is less than or equal to 0.32 mA at 2.0 VDC and source
current is less than or equal to 12 mA at 0.8 VDC.
C.1.9 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:
Two fully functional synchronous and asynchronous serial ports
DB-25 connectors
Increased baud rate speed (to 384 Kbaud, synchronous; 460.8 Kbaud,
asynchronous)
Variable edge rate for greater performance
EBus interface
C.1.9.1 Serial Port Components
Serial port components include a serial port controller, line drivers, and the line
receivers.
The serial port controller contains 64-byte buffers on both the input and output. This
enables the serial port to require less CPU bandwidth. Interrupts are generated when
the buffer reaches 32 bytes or half full. The serial port controller contains its own
crystal oscillator that supports rates of up to 921.6 Kbaud.
The line drivers and line receivers are compatible with both RS-232 and RS-423. Two
system board jumpers are used to set the line drivers and line receivers to either
RS-232 or RS-423 protocols. The line driver slew rate is also programmable. For baud
rates over 100K, the slew rate is set to 10 VDC/msec. For baud rates under 100K, the
slew rate is set to 5 VDC/msec.
Functional Description 263