Elecraft KRC2 Manual - Manual (Page 29)

29
Circuit Details
Refer to the schematic diagram, Figure 7 on page 29.
U1 is a PCI 16F877A microcontroller. This device controls the operation of the KRC2. U1
operates at a clock frequency of 4 MHz, as determined by X1. It communicates with relay drivers
U4-U7 using its built-in Serial Peripheral Interconnect (SPI) interface, commanding them to turn
their outputs on or off. Which outputs should be on or off is determined during execution of the
microcontroller’s internal program. It uses band input from the attached radio or computer to
determine which driver output(s) should be enabled.
The MCU receives band information from the attached radio through one of six input channels.
Three channels are analog, using the MCU’s built-in analog to digital converters to measure band
voltages, while the other three are digital in nature. These consist of a BCD parallel input and two
serial inputs. One serial input enables the MCU to participate on the K2’s AuxBus channel, while
the other uses the MCU’s built-in UART serial port to communicate using the Elecraft serial
protocol (similar to the Kenwood serial protocol). Selection of the active input method is made by
setting the W1-W3 jumpers.
The UART serial port uses industry-standard RS-232 voltages. These are switched in the KRC2
using U2, a MAX1406 (or similar) RS-232 interface. U2 requires a negative power supply for its
operation. This is developed using the oscillator and rectifier circuitry of Q1. Q1 and X2 and their
associated components form an oscillator running at 18.289 MHz. Output from this oscillator is
fed to a voltage-doubling rectifier made up of D6, D7, and C15, then fed to U2’s –12V input pin.
The circuit typically provides –7.5 volts, well within the range required by the RS-232
specification.
For analog band operation, the input voltage is connected to the microcontroller’s AN1, AN2 or
AN3 inputs through divider or isolation networks. The AN1 input comes from pin 9 of J1 or J2,
depending on jumper selection. It is reduced by the divider network made up of R6 and R10. The
values for these resistors provide a division by 2, giving the input a maximum range of 0 to +10
volts. The AN3 input is identical to that of AN1, with RR8 and R11 making up the divide-by-2
network. This input comes from J1 or J2 pin 4. AN2 comes from J1/J2 pin 7, and is fed directly to
the MCU input without a voltage divider. Its input range is 0 to +5V.
The KRC2 can communicate with two devices using the RS-232 port, depending on the setting of
S5. In S5’s normal operation position, the KRC2 listens directly to the transceiver’s serial output,
and also can send directly to the radio attached to the XVCR port. Additionally, the RS-232
signals from a computer attached to the PC port (J1) are ORed with the KRC2’s serial port,
allowing it to send, and listen, to the transceiver as well. No protection is provided to keep
transmissions from the KRC2 and PC from colliding on this channel, so care must be taken to
prevent this in the PC and KRC2 operation. The S5’s Download (DL) position enables the
microcontroller to communicate directly with a PC. This is used in downloading configuration and
firmware upgrades to the KRC2.