Hardware Description; Mm3 Design - GE MM3 Instruction Manual

3.3 HARDWARE DESCRIPTION

3 HARDWARE
3.3 HARDWARE DESCRIPTION

3.3.1 MM3 DESIGN

A 16 bit 68HC16 microcontroller IC performs program execution and control logic for the MM3. 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 a 512K × 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
communications channel on the main CPU board. The display, a 4 × 4 keypad and the front panel LEDs data
are exchanged via modbus data packets.
External switch inputs are driven with the same voltage applied to the control voltage 120 or 240 V AC which
triggers an optocoupler for isolation. All control logic based on the state of these inputs determines operation of
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up to 4 output relays which are also driven from a latch under program control. Like the inputs, the relay out-
puts are driven from an isolated 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 sig-
nals. Separate AC inputs for phase 1, phase 2, phase 3, and ground fault signals are sampled at a 1.67 ms
rate, squared and summed. RMS current is then determined by deriving the square root of the sampled wave-
form 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, control supply voltage and internal reference voltage
are also monitored. An external precision 5 V DC reference is used as the input reference for the A/D con-
verter.
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 MM3 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 feature.
Serial communications at up to 57600 baud is implemented with a UART. All necessary timing and control is
performed inside the chip. An external transceiver chip converts the digital data to an RS485 interface. Direc-
tion, 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 MM3 can be selected as 120 or 240 V AC using a switch and dual wound pri-
mary transformer. A filter is incorporated between the incoming supply and transformer primary to prevent tran-
sients 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 exe-
cution. 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 conditions. Should normal program execution
fail, the 68HC705 resets the main CPU.
3-6 MM3 Motor Manager 3 GE Power Management