Theory Of Operation; Hardware Description - GE MOTOR MANAGER 2 Instruction Manual

3.3 THEORY OF OPERATION

3 HARDWARE
3.3 THEORY OF OPERATION

3.3.1 HARDWARE DESCRIPTION

A 16 bit 68HC16 microcontroller IC performs program execution and control logic for the MM2. Refer
to the block diagram for a complete overview of the MM2 circuitry. It has an 8 or 16 bit bus width
which can be selected dynamically with each external memory fetch allowing a mix of 8 and 16 bit
devices. Internal clock rate is 16 Mhz. Instructions are stored in two 128K × 8 bit flash memory, data
is stored in an 32K × 8 RAM while Setpoints and accumulated data are stored in a 8K × 8 EEPROM.
An intelligent display module with its own microprocessor, memory and command set is accessed
through a buffer on the data bus. The display, a 4 × 4 keypad and the front panel LEDs are multi-
plexed through the same buffer.
External switch inputs are driven with 120 VAC which triggers an optocoupler for isolation. All control
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logic based on the state of these inputs determines operation of up to 4 output relays which are also
driven from a latch under program control. Like the inputs, the relay outputs are driven from an iso-
lated power supply and optocoupler to prevent switching transient energy from affecting the CPU.
A 10 bit successive approximation A/D on the 68HC16 CPU with 8 channels is used to measure all
analog signals. Separate AC inputs for phase 1, phase 2, phase 3, and ground fault signals are sam-
pled at a 1.67 ms rate, squared and summed. RMS current is then determined by deriving the
square root of the sampled waveform over several cycles. The sampling time is set to measure an
integral number of cycles to reduce the affects of noise and harmonics. Thermistor, analog input, VT
voltage, control supply voltage and internal reference voltage are also monitored. An external preci-
sion 5V DC reference is used as the input reference for the A/D converter.
When power to the unit is removed, a small 8-bit processor (68HC705) will continue to operate for a
period of at least 1 hour. This processor is powered from a large backup capacitor. The 68HC705
accurately measures the time that the MM2 has been without control power. When power is re-
applied the main processor will read the time off from the small processor and then very accurately
calculate the thermal capacity value. This time off value is also used for the undervoltage restart fea-
ture.
Serial communications at up to 19200 baud is implemented with UART circuitry internal to the
68HC16 microcomputer. All necessary timing and control is performed inside the chip. An external
transceiver chip converts the digital data to an RS485 interface. Direction, receive data and transmit
data are on the input side with a two wire twisted pair driver on the output.
AC control voltage to power the MM2 can be selected as 120 or 240 V AC using a switch and dual
wound primary transformer. A filter is incorporated between the incoming supply and transformer pri-
mary to prevent transients from affecting the circuitry.
Separate, isolated secondary supplies are used for CPU power, I/O and communication drivers.
Optocoupling and transformer coupling are used between isolated circuits to prevent transients from
upsetting program execution. The 68HC705 is used to provide separate watchdog timer and power
fail monitoring control to ensure that the main CPU starts and operates under any input voltage con-
ditions. Should normal program execution fail, the 68HC705 resets the main CPU.
3-6 MM2 Motor Manager 2
GE Power Management