Of the features outlined above, the operation of the two bar graph displays is the central function of the ULA in the IP.

The block diagram shows how this is achieved. Each display is fed from its own counter/decoder. An internal multiplexer is used to timeshare the analogue portions of the ULA comprising digital to analogue converter ( DAC) and front end signal conditioning and level comparator. The multiplexer runs at a variable rate dependent on the conversion time of the motor load current display. A typical sequence is as follows:

1) Multiplexer switches to update the motor load display.

2) Counter 2 is reset producing zero output from the DAC on to the negative input of the comparator.

3) The comparator output is high allowing the internal clock to start counting up, counter 2.

4) The DAC output steps up until it equals or exceeds the analogue input on the + of the comparator - this voltage is a function of the motor load current.

5) The comparator output changes state and the conversion sequence finishes. The internal multiplexer changes over to update the fuel gauge display.

6) The analogue voltage applied to the comparator +ve is a function of the compensated battery voltage.

7) The state (count) of the fuel gauge counter is applied to the DAC and hence to the comparator.

8) capitalise first the DAC voltage is higher than the measured voltage, the resettable delay timer starts.

9) If the delay times out, the fuel gauge display counter is decremented by one count and the process continues.

10) The delay timer is controlled by the multiplexer so that it only responds to the comparator output when the fuel gauge cycle is complete.

In summary, the ULA circuitry is multiplexed to provide sample and held display of motor current and a decrementing display of compensated battery voltage - besides the other functions mentioned in the first section.

Figure 2 - ULA Front End Circuitry

The drawing shows a section of the CB as well as the ULA it internal circuits. An understanding of the relationships between the voltages and currents is necessary if any changes of calibration or application on intended.

Starting with the motor current display mode; the analogue multiplexer switches to the "current" position. The emitter current of transistor 3, I4 is defined by:

    I4=V/R9

The current is steered by the multiplexer switch and flows from the Vref rail, through R2 and R18. It produces a voltage drop at pin 27 with respect to pin 23 equal to:

    I4(R2+R18)

The DAC is fed with a reference current I5. This is defined by the relationship:

    I5=(Vref-1.2)/R5

The least significant bit of the DAC is defined by a:

    LSB=I5/I4

if the counter driving the DAC steps up until the DAC output current, Idac, flowing through R6 from the the Vref rail, produces a voltage drop across R6 at pin 26 equal to or greater than that at pin 27. At this point the comparator output changes and the cycle stops. The ULA multiplexer then switches to fuel gauge mode.

In fuel gauge mode, the following events happen. The analogue multiplexer is switched to the "fuel" position. An offset current I6 flows from Vref pin 23 through VR1 and R3 into pin 2 which is at a constant potential of 2 Vbe, i e about 1.2 volts.

    I6=(Vref-1.2)/(R3+VR1)

A current proportional to motor current, i e I4 is subtracted from the offset current leaving a nett current I3 flowing into the current mirror.

    I3=I6-I4

A current I1 proportional to battery voltage flows through R17.

    I1=(Vbatt-0.6)/R17

The mirror circuit sinks a current I2 defined by:

    I2=0.5(I1-I3)

Current I2 flows from Vref through R2 producing a offset voltage on pin 27 of the comparator. The comparator output will let the delay timer starts if the DAC output current causes pin 26 to be offset more than pin 27. This will cause the fuel bar graph to eventually decrement by one segment, reducing the DAC output current. Note from equations 2 and 4 that: if

    I2=0.5(I1-I6+I4)

This indicates that the fuel gauge reading (its proportional to I2) if is higher if I1 is higher, is offset by I6 and increased by I4 - hence the current compensated offset voltage function for fuel is achieved.

Note that the internal voltage reference at Vref is fed from the +5VB rail via resistor R1.

Figure 3 - Instrument Pod

The Ferranti ULA (uncommitted logic array) performs all the key functions. It is powered from a 5 volt supply +5VB derived from the C5 12 volt battery via regulator TR3. This supply is always present so that the state of the fuel gauge is effectively "remembered".

The ULA drives the two bar graphs through a common five line multiplexed bus - SD1 to SD5. TR1 and TR2 provide anode multiplexing. These transistors should have a saturation voltage of less than 0.5 volts at 100 mA collector current and 3.75 mA base current. The FST290 is a specially selected transistor to this specification.

The ULA contains three internal oscillators connected to pins 7, 9 and 10. The approximate frequencies and functions are as follows:

• R10 and C6: 0.4 Hz divided by eight to give 20 second fuel gauge delay timer.
• R7 and C4: 50 Hz divided internally to provide flash rate and fuel gauge lock out time.
• R11 and C7: 9.5 KHz divided by 2 to drive piezo buzzer and internal multiplexer.

A piezo-ceramic buzzer is a.c. coupled to pin 20 to act as the audio warning device.

The +5VA supply is obtained from the regulator in the CB. This supply is only present when the C5 is powered up, thus the displays are only lit in this condition.

The motor switch causes a 12 volt signal routed via a 15 K resistor in the CB to be applied to the ULA pin 21. The ULA drives the motor relay in the CB via a transistor, from pin 13.

The motor temperature thermistor is arranged in a potential divider network across the +5VB supply together with R9 and R20. The ULA pins 5 and 6 are comparator inputs. The threshold voltage on the comparators is set within the ULA to 0.5 Vcc. The two trip levels operate with nominal thermistor resistances of 1100 and 900 Ohms.

The resistor networks associated with pins 1,2,23,24,26,27 and 28 control the analogue front end of the ULA for voltage and current measurement. The pot VR1 is used to trim the effective value of the internal voltage reference and hence to set up the "battery offset". This is the effective no-load battery voltage at which battery cut-off occurs.

ULA pins 3 and 4 are associated with a remote motor current sensor in the CB. The display of motor load current and compensation for battery internal impedance is controlled by the emitter current of the ULA transistor on pin 3 and 4.

Figure 4 - Control Box

The connector X9 joins to X1 in the IP on a pin for pin basis. X6 provides system inputs for the motor temperature thermistor (pins 1 and 2), the motor over-temperature trip (pins 4 and 5) and a means of providing remote cut-out (pins 7 and 8).

Transistor TR3 drives the relay. The zener diode ZD1 suppresses the transient produced by the relay coil at turn-off. The rapid decay of coil current which the zener produces is vital to minimising the arc on the relay contacts which is commutated by diode D1.

The regulator IC1 provides 5 volts to the IP display and to TR1 etc.

The op-amp IC2 provides two functions. Firstly it converts the motor current shunt signal (1mV/amp) between X10 and X11 into a current sink in conjunction with the ULA transistor on pins 3 and 4. The value of the current is determined by R9. Resistor R8 and C4 provide a filter. Secondly, IC2 provides an amplifier to drive the regenerative pair TR1/TR2 to detect stall. The amplifier provides gain of 100/15 and the output is integrated by C3. If TR2 turns on, which requires about 0.6 volts on IC2 pin 1, TR1 also turns on and latches up TR2, irrespective of IC2 pin 1. The 5 volt signal on the collector of TR1 feeds via D2 and R13 and simulatea a very low value of motor thermistor. The IP interprets this as an overheat and locks-out the relay drive. This stall condition can only be reset by removing the +5VA supply briefly.

Application of C5 instrumentation system to other vehicles

Thermal protection - different values of thermistor can be used by changing R9 and R20 in the IP. The trip voltage levels of the ULA comparator have already been discussed.

Motor load indicator - the calibration of this display is defined by the sum of R2 and R18 in the IP. Also by the current flow in R9 (CB) for a given shunt current.

Stall protection - by varying R6 and R7 (CB) the sensitivity of the stall protection can be changed, whilst C3 controls the time delay.

24 volt systems - with suitably specified components the C5 circuits can be used on a 24 volt system. Changes required are as follows:

Instrument pod

• C 12 - increase voltage rating
• R 17  - double to 94 KOhms for correct fuel gauge operation on a series connected pair of batteries.

Control box

• C2 - increase voltage rating
• IC1 - increase heat sink size
• Relay -  add series resistor

Alternatively, the Instrumentation can be run from 12 volts as normal except for the battery sensing input to the ULA via R17.

Reliability

Static discharge can damage the ULA. It is also possible for the LM358 amplifier in the control box to fail. Some simple modifications have been devised to overcome both these shortcomings. They are as follows:

Instrument pod

• Connect a 47 K resistor in series with the track from X1 pin 6 to IC1 pin 3
• Connect catch diodes (IN914) to Vcc and GND on IC1/4

Control box

• Derive power for IC2 (Pin 8) from the +5VA supply instead of the +12v

Instructions for setting up the offset pot VR1 in the instrument pod

Firstly the internal multiplexer must be disabled and forced into fuel gauge mode. This occurs if pin 9 is grounded at power up. The steps are:

1) connect ULA pin 9 to pin 22 (GND)

2) apply the required battery lock out voltage to +BATT (top of R17). The suggested value is 10.5 volts

3) apply +5V to Vcc pin 8

4) set I4 to zero by grounding pin 3 for example

5) using a high impedance digital volt meter (z greater than one megohm) measure the voltage between pin 23 (+) and pin 27 (-)

6) set VR1 for +200 millivolts. If out of range then R3 should be increased by, say 1000 ohms

7) turn-off power, remove the short on pin 9 and check fuel gauge operation under normal conditions. Note that with the nominal component values as shown, drop out time per segment is about 20 seconds with two hidden segments beyond the most significant bit.