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PCB605 Application Board 3250/4250 Circuit Description
Input/Output Map: The 64 kilo-bytes of I/O address space control access to all peripherals
such as the serial ports, application board, etc. The processor uses 256 bytes, called the
Peripheral Control Block, to control the integrated serial ports, timers, interrupts, chip selects,
and I/O pins. The Peripheral Control Block is fully described in the 80C186EB/80C188EB
Microprocessor User’s Manual. The application board’s registers and the CPU board’s
registers control access to various functions and are described later.
Memory: Two types of memory are avail able: read-only and read/write. The chips used
here have 8-bit wide data buses. Used in pairs, the memory becomes compatible with the
processor’s 16-bit wide data bus. The read-only memory is implemented using two 128 kilo-
bytes x 8 bits Flash EPROMs. Flash memory is only programmable when a programming
voltage (in this case, 12 volts DC) is applied to the memory; otherwise, it behaves like read-
only memory. The boot code controls the programming voltage, since this is the only place
where new software can be downloaded. The read/write memory is implemented using two
128 kilo-bytes x 8 bits static RAMs.
Watchdog Timer: The watchdog timer provides a mechanism to reset the processor when
the software is not behaving normally. The software “pets” the watchdog every
100-200 milliseconds. As long as the watchdog is “petted”, nothing happens. However, if the
software stops “petting” the watchdog, the watchdog “bites” the processor by activating
the processor’s RESET pin, causing the processor to reset. The watchdog will also reset the
processor if the power supply falls below 4.75 VDC. These two features allow the instrument
to recover from abnormal software and hardware conditions.
Real Time Clock (RTC): The real time clock maintains the current date and time. The
RTC uses an internal clock circuit with an internal, 10-year life, lithium battery to perform
timekeeping. The battery also preserves the contents of internal memory.
Dip Switch: The three-position or four-position dip switch allows the user to download new
software via one switch. The other two switches are available for selecting different operating
modes. These operating modes and the process for downloading software are described later
in this manual.
Voltages: Three voltages are present on the CPU board: +5 VDC, +12VDC, and V
PP
.
• +5 VDC powers all the logic.
• +12 VDC is switched on and off to create the flash memory’s programming voltage, V
PP
.
Glue Logic: The glue logic performs the functions of creating the RAM memory chip selects,
the Flash memory write signals, the V
PP
control signal, the watchdog control signal, and
accessing the dip switch. The CPU board has two software accessible hardware registers to
read the dip switch and to control watchdog and V
PP
.
Connectors: A 64-pin connector provides address, data and interrupt signals. A 16-pin
connector provides general chip selects and serial receive/transmit signals. The application
PCB uses a subset of the available signals.
PCB605 Application Board
The application PCB contains all circuitry specific to the instrument. The board includes
voltage supplies, indicators, analog-to-digital converters, drive circuitry, parallel ports, serial
ports, and various switches.
Analog-to-Digital Converters: Two analog-to-digital converters are provided; one for the
sample thermistor probe, and one for the block thermistor probe. The thermistors vary in
resistance from approximately 2 kilo-ohms at room temperature, to approximately 10 kilo-
ohms at -12ºC. Typically, a 0.6 ohm change in the thermistor’s resistance equates to a 1
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