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Document: LT0273 MX4428 MXP Engineering / Technical Manual
MXP Technical Description
Issue 1.5 24 March 2006 Page 7-3
7.2 CIRCUIT DESCRIPTION
7.2.1 BLOCK DIAGRAM
A block diagram of the MXP is given in Figure 7.1.
The MXP can be divided into 4 sections:
(i) The microprocessor and memory. This is the "heart" of the MXP.
(ii) The power supply. The power supply produces the 40V isolated supply for the
Analogue Loop and also the 5V isolated supply for the microprocessor.
(iii) The MX4428 Loop Interface. The Loop Interface contains the connect, disconnect
circuitry for the MX4428 Loop Power Supply and also the serial data driver circuits.
(iv) The Analogue Loop Interface. This section contains the Loop Driver/ Receiver circuit
and the Loop Isolator circuits.
7.2.2 MICROPROCESSOR & LOGIC CIRCUITRY
The MXP is controlled by the 68302 Microprocessor CPU (U1). Connected to the CPU bus
is the FLASH (U2) which contains the MXP software, and the RAM (U3 and U4) which is
used for storing parameters and data associated with devices on the Analogue Loop.
The 68302 includes a communications processor with 3 serial ports. These are used for the
2 responder loop ports and a diagnostic port.
To reduce the number of components on the PCB, a PAL (Programmable Array Logic) (U5)
is used to generate bus signals such as RD-, L WR-, and U WR-. It also drives the status
LED and is used to read some of the DIP switches.
Both the FLASH (U2) and PAL (U5) are factory programmed for use in the MXP. However
the FLASH (U2) can easily be reprogrammed in the field.
7.2.2.1 Power On Reset & Watchdog Circuits
The power on reset consists of the DS1232 (U6), which drives the RST signal to the CPU
low when the 5V supply is below 4.6 Volts. This ensures that the CPU does not corrupt the
RAM when the 5V supply collapses, and the CPU starts up reliably when the 5V supply turns
on. The DS1232 also includes a Watchdog circuit inside, to produce a RESET if the CPU
stops running properly.
7.2.2.2 Memory
Memory addresses are decoded by the MC68302. When first powered up the FLASH
occupies the bottom 8k bytes. However the software relocates the FLASH so that the
memory addressing is as follows -
000000 - 03ffff RAM
400000 - 47ffff FLASH
800000 - 80000f LED and SWITCHES via PAL
f00000 - f00fff INTERNAL RAM AND PERIPHERALS
The RAM memory and PAL chip are powered by the +5VB supply, which has a supercap
(C68) to supply power when the MXP is powered down. This allows the MXP to retain its
RAM memory contents for typically 10 hours on MXP power down.