Somnath Pal -
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RAMS of a Vital Input Card
CHAPTER -1
Introduction
For a Railway Signal Interlocking system, Field Input conditions to be monitored by the Interlocking equipment are Track Cct / Axle Counter, Point Detection, Signal Aspect Proving, Level Crossing Gate, Crank Handle and Panel Buttons. The monitoring is done by reading the Pick-up or Drop contacts of corresponding Relays.
This could have been done by operating a Transistor through external Voltage fed via the pick-up Contact of the Relay. The Output of Transistor is processed by the Logic Solver ( Processor). But in that case, there would be no Isolation between external Analog Input and internal Digital Output circuits. An Opto Coupler is used for this purpose, where the Input LED and Output Phototransistor are electrically isolated . The Input through Pick-up contact of Relay operates the LED of the Opto Coupler, light from which turns the output Phototransistor. The interfacing using a Transistor and an Opto coupler is showed in Fig 1(a)&(b)
When the Relay is in Pick-up condition, the Transistor is Forward biased and Collector Output to Processor is ‘0’ . During Drop condition of the Relay, Collector Output is ‘1’ .
Input Interface Card converts the Analog information of DC Voltage fed through the Relay contacts to send TTL level to Processor. The basic features of the Card are:
All Inputs are fed through Opto Couplers having a minimum Isolation Voltage of 1500V.
All Inputs are protected from Transient Voltages and Surges by using Varistors.
To reduce Single-point Failure, both Front and Back potential-free contacts of each external Relay are interfaced. They are read through different Ports and Bits to avoid common-mode failure.
Each Card gets Hardware Address of the particular Slot of the Backplane, to which it is inserted. This Address must be matched with the Software Address sent by Logic Solver Card before the external Relay Inputs are read.
Input Cards can be inserted or extracted with System Power-ON conditions. A special circuit is used to prevent transient to Stable +5V Supply to ICs,
Input data are read employing Input Toggle by the logic Solver or using multiple sets of Opto Couplers ensuring Hardware redundancy.
The Input Data Structure of the Interlocking System Inputs is given in Fig 2:
Panel Buttons and Keys are called Non-Vital Inputs and a Non-Vital Input / Output Card is used to read 32 Inputs . All other Inputs are Vital Inputs and a Vital Input Card reads 16 Relay Inputs.
A detailed diagram is showed in Fig 3 . Since Railway Interlocking is a Safety-critical System, Inputs from both Pick-up and Drop Contacts of a Relay is analysed.
Each Input is monitored by Opto Couplers by supplying Current from an Isolated 12V Supply .
The Input to Processor will be ‘ 01’ in Drop Condition and ‘ 10’ in Pick up Condition . These two combinations are Valid Inputs . Other two Inputs ‘ 00’ and ‘ 11’ are Invalid and Processor will show Fault. But during transition of Relay between Drop to Pick-up or Pick- up to Drop , both the Opto Couplers will be OFF . So, Output will be ‘11’ , which is Invalid
A De-bouncing circuit (U1) is used for feeding steady Input to the Logic Solver. In this case, during transition of Relay, the Input to Processor will remain unchanged from Last State.
When the Relay X is not operated , current flows through the Drop Contact , Inductor, L1, Current Limiter R1, Resistor R2 and input Diode of Opto Coupler OC1. The Opto Coupler conducts and a level ‘ 0 ’ is fed to Pin1 of U1 . This makes output Pin 3 of U1 as ‘ 1 ’. At the same time, since no current passes through Input Diode of OC2, it does not conduct. Level ‘1’ is fed to Pin 5 of U1 and output Pin 6 of U1 to Port becomes ‘0’.
Thus we get a pattern 0101 at the Port.
When the Relay X is operated , current flows through the Pick-up Contact , Inductor L2, Current Limiter R5, Resistor R6 and input Diode of Opto Coupler OC2. The Opto coupler conducts and a level ‘0 ’ is fed to Pin 5 of U1 . This makes output to Pin 6 of U1 as ‘0’ . At the same time, since no current passes through Input Diode of OC1, it does not conduct. Level ‘1’ is fed to Pin 1 of U1 and output to Pin 3 of U1 to Port becomes ‘0’ .This time, the inputs to Port changes to 1010 .
We find that Port inputs 0101 and 1010 are the valid levels . All other combinations are invalid levels.
If Hardware De-bounce Cct U1 is replaced by Software De-bounce to reduce Parts Count to enhance Reliability, the inputs to Port will change to 01 when Relay X is not operated and 10 when it is operated.
If a vital decision is taken by Pick-up condition of Relay X, then a failure to read Pick-up condition as Drop, is a Safe failure . But if Drop condition is read as Pick-up condition, it will be Unsafe failure .
Now we shall have to do an Failure Mode Effects & Criticality Analysis (FMECA) study of the Cct showed in Fig 3 and identify the critical component to cause Unsafe Failure and mitigate the Hazard.
Failure Mode Effects & Criticality Analysis (FMECA)
We shall first study the failure modes of the components. For every failure mode of components , we shall their effect for Drop and Pick-up conditions.
Typical faults of an Electronic component are Open Cct, Short Cct, Drift in Parameter and Functional Faults. Average relative frequencies of Failure Modes of various Components are given in the Table 1 below.
Terminating Resistors and Current Limiting Resistors are of mainly Metal Film type for which Short cct Mode is incredible and hence are not considered. When Resistors are used in Nodes with Pull up Totem-pole Driver , even Drift in value is not to be considered. For open Collector Modes like Opto coupler output, the design ensures much higher Collector current as well as Diode input current , so that even ± 20% Drift does not affect the Circuit behaviour.
So, for Resistors , only Open Cct Mode is considered.
For Inductors, any Drift will not affect normal operation and only Open Mode of failure is considered.
For Varistor, Open Mode does not affect normal operation and if the Clamp Voltage is chosen much higher than the required Value , Drift also does not have any effect. Only Short cct Mode is analysed .
For ICs, both s-a-0 and s-a-1 Faults are considered for every Pin.
Now we will consider component-wise FMECA. The effects during Relay Drop and Pick-up conditions are analysed for each Component Failure.
a) If Inductor L1 is Open
Relay Drop condition:
Current does not pass through input of Opto Coupler OC1. U1 pin 1 is at ‘1’ and U1 pin 5 is aIso ‘1’. Output to Port will be ’1011’. Fault is detected as Invalid Data and failure is Safe. Since both the Inputs to Debounce Cct are at ‘1’, its Output will remain at last State, i.e. 01.
Relay Pick-up condition :
Current passes through input of Opto Coupler OC2. U1 pin 1 is at ‘1’ and U1 pin 5 is at ‘0’. Output to Port will be ’1010’. Fault is not detected and failure is Safe.
b) If Inductor L2 is Open
Relay Drop condition:
Current does not pass through input of Opto Coupler OC2. U1 pin5 is at ‘1’ and U1 pin 1 is at ‘0’. Output to Port will be ’0101’. Fault is not detected and failure is Safe.
Relay Pick-up condition :
Current passes through input of Opto Coupler OC1. U1 pin 1 is at ‘1’ and U1 pin 5 is also at ‘1’. Output to Port will be ’1011’. Fault is detected as invalid data.
c) if Metal Oxide Varistor RV1 is Short :
Relay Drop condition:
Current does not pass through input of Opto Coupler OC1. U1 pin 1 is at ‘1’ and U1 pin 5 is aIso ‘1’. Output to Port will be ’1101’. Fault is detected as invalid data and failure is Safe.
Relay Pick-up condition :
Current passes through input of Opto Coupler OC2. U1 pin 1 is at ‘1’ and U1 pin 5 is at ‘0’. Output to Port will be ’1010’. Fault is not detected and failure is Safe.
d) if Metal Oxide Varistor RV2 is Short :
Relay Drop condition:
Current does not pass through input of Opto Coupler OC2. U1 pin5 is at ‘1’ and U1 pin 1 is at ‘0’. Output to Port will be ’0101’. Fault is not detected and failure is Safe.
Relay Pick-up condition :
Current does not pass through input of Opto Coupler OC2. U1 pin 5 is at ‘1’ and U1 pin 1 is aIso ‘1’. Output to Port will be ’1011’. Fault is detected as invalid data and failure is Safe.
e) If Current Limiting Resistance R1 is Open:
Relay Drop condition:
Current does not pass through input of Opto Coupler OC1. U1 pin 1 is at ‘1’ and U1 pin 5 is aIso ‘1’. Output to Port will be ’1101’. Fault is detected as invalid data and failure is Safe.
Relay Pick-up condition :
Current passes through input of Opto Coupler OC2. U1 pin 1 is at ‘1’ and U1 pin 5 is at ‘0’. Output to Port will be ’1010’. Fault is not detected and failure is Safe.
f) If Current Limiting Resistance R2 is Open:
Relay Drop condition:
Current does not pass through input of Opto Coupler OC1. U1 pin 1 is at ‘1’ and U1 pin 5 is aIso ‘1’. Output to Port will be ’1101’. Fault is not detected data and failure is Safe.
Relay Pick-up condition :
Current does not pass through input of Opto Coupler OC2. U1 pin 1 is at ‘1’ and U1 pin 5 is also at ‘1’. Output to Port will be ’1011’. Fault is detected as invalid and failure is Safe.
g) If Current Limiting Resistance R5 is Open:
Relay Drop condition:
Current does not pass through input of Opto Coupler OC2. U1 pin 1 is at ‘0’ and U1 pin 5 is aI ‘1’. Output to Port will be ’0101’. Fault is not detected data and failure is Safe.
Relay Pick-up condition :
Current does not pass through input of Opto Coupler OC2. U1 pin 1 is at ‘1’ and U1 pin 5 is at ‘1’. Output to Port will be ’1010’. Fault is not detected and failure is Safe.
h) If Current Limiting Resistance R6 is Open:
Relay Drop condition:
Current does not pass through input of Opto Coupler OC2. U1 pin 1 is at ‘0’ and U1 pin 5 is aI ‘1’. Output to Port will be ’0101’. Fault is not detected data and failure is Safe.
Relay Pick-up condition :
Current does not pass through input of Opto Coupler OC2. U1 pin 1 is at ‘1’ and U1 pin 5 is at ‘1’. Output to Port will be ’1010’. Fault is not detected and failure is Safe.
i) If Protection Diode D1 is open:
Normal operation of the cct is not affected. But a reverse voltage of more than 6V during Drop condition would damage the Diode of Opto coupler OC1. During Pick-up condition, there is no effect.
j) If Protection Diode D2 is open:
Normal operation of the cct is not affected. But a reverse voltage of more than 6V would damage the Diode of Opto Coupler OC2 during Pick-up. There is no effect during Drop condition.
k) If Protection Diode D1 is short:
Relay Drop condition:
Opto coupler OC1 will be OFF since current will be bypassed via D1. U1 pin 1 is at ‘1’ and U1 pin 5 is at ‘1’. Output to Port will be ’1101’. Fault is detected as invalid data and failure is Safe.
Relay Pick-up condition :
Opto coupler OC1 will be OFF since current will be bypassed via D1. U1 pin 1 is at ‘1’ and U1 pin 5 is at ‘1’. Output to Port will be ’1010’. Fault is not detected failure is Safe.
l) If Protection Diode D2 is short:
Relay Drop condition:
Opto coupler OC2 will be OFF since current will be bypassed via D2. U1 pin 1 is at ‘1’ and U1 pin 5 is also at ‘1’. Output to Port will be ’ 0101’. Fault is not detected and failure is Safe.
Relay Pick-up condition :
Opto coupler OC2 will be OFF since current will be bypassed via D2. U1 pin 1 is at ‘1’ and U1 pin 5 is aIso ‘1’. Output to Port will be ’1011’. Fault is detected as invalid data and failure is Safe.
m) If Input Diode of Opto Coupler OC1 is open:
Relay Drop condition:
Current does not pass through input of Opto Coupler OC1. U1 pin 1 is at ‘1’ and U1 pin 5 is aIso ‘1’. Output to Port will be ’1101’. Fault is detected as invalid data and failure is Safe.
Relay Pick-up condition :
Current does not pass through input of Opto Coupler OC1. U1 pin 1 is at ‘1’ and U1 pin 5 is at ‘0’. Output to Port will be ’1010’. Fault is not detected and failure is Safe.
n) If Input Diode of Opto Coupler OC2 is open:
Relay Drop condition:
Current does not pass through input of Opto Coupler OC2. U1 pin 5 is at ‘1’ and U1 pin 1 is at ‘0’. Output to Port will be ’0101’. Fault is not detected and failure is Safe.
Relay Pick-up condition :
Current does not pass through input of Opto Coupler OC2. U1 pin 1 is at ‘1’ and U1 pin 5 is also at ‘1’. Output to Port will be ’1011’. Fault is detected as invalid data.
o) If Collector in Opto Coupler OC1 is open:
Relay Drop condition:
Due to the pull up Resistor 5, Current does not pass through input of Opto Coupler OC1. U1 pin 5 is at ‘1’ and U1 pin 1 is at ‘0’. Output to Port will be ’0101’. Fault is not detected and failure is Safe.
Relay Pick-up condition :
Opto Coupler OC2 starts conducting and port input becomes 0101. Fault is not detected and failure is Safe.
p) If Collector in Opto Coupler OC2 is open:
Relay Drop condition:
Current does not pass through input of Opto Coupler OC2. U1 pin5 is at ‘1’ and U1 pin 1 is at ‘0’. Output to Port will be ’0101’. Fault is not detected and failure is Safe.
Relay Pick-up condition :
Current passes through input of Opto Coupler OC1. U1 pin 1 is at ‘1’ and U1 pin 5 is also at ‘1’. Output to Port will be ’1011’. Fault is detected as invalid data.
q) If Collector and Emitter of OC1 is short:
Relay Drop condition:
We get 1 in Pin 1 of U1 and port gets data 0101. Fault is not detected and failure is Safe.
Relay Pick-up condition :
Both pins 1 and 5 get 1 and we get 0110. Fault is detected as invalid data.
r) If Collector and Emitter of OC 2 is short:
Relay Drop condition:
We get 1 in Pin 5 of U1 and port gets data 1010. Fault is not detected and failure is Safe.
Relay Pick-up condition :
Both pins 1 and 5 get 1 and we get 0110. Fault is detected as invalid data.
s) If Pull up Resistor R3 is open:
Relay Drop condition:
Since Opto Coupler OC1 is in operated condition, we get 0 in Pin1 of U1 and port gets data 0101. Fault is not detected and failure is Safe.
Relay Pick-up condition :
If there is large leakage current from Supply and Ground, the Collector of Opto coupler OC1 will be at level ‘0’. So data to port will be 0110. Fault is detected as invalid data.
t) If Pull up Resistor R7 is open:
Relay Drop condition:
Since Opto Coupler OC2 is in operated condition, we get 0 in Pin5 of U1 and port gets data 0110. Fault is detected as invalid data and failure is Safe.
Relay Pick-up condition :
If there is large leakage current from Supply and Ground, the Collector of Opto coupler OC1 will be at level ‘0’. So data to port will be 1010. Fault is not detected.
u) If S-a-0 fault is at Pin 1 of U1:
Relay Drop condition:
Pin 3 of U1 will be ‘1’ and Port Input is 0101. Fault is not detected.
Relay Pick-up condition :
Port Input will be 0110, which is invalid.
v) If S-a-0 fault is at Pin 5 of U1:
Relay Drop condition:
Pin 6 of U1 will be ‘1’ and Port Input is 0110 which is invalid.
Relay Pick-up condition :
:Port Input will be 1010. Fault is not detected.
w) If S-a-1 fault is at Pin 1 of U1:
Relay Drop condition:
Pin 1 and Pin 5 of U1 both will be ‘1’ and Port Input remains 0101. Fault is not detected.
Relay Pick-up condition :
Port Input will be 1010. Fault is not detected.
x) If S-a-1 fault is at U1-5:
Relay Drop condition:
Pin 6 of U1 will be ‘0’ and Port Input is 0101. Fault is not detected.
Relay Pick-up condition :
Pin 1 and Pin 5 of U1 both will be ‘1’ and Port Input remains 1010. Fault is not detected.
y) If S-a-0 fault is at U1-3:
Relay Drop condition:
Pin 3 of U1 will be ‘0’ and Port Input is 0011, which is invalid.
Relay Pick-up condition :
Port Input is 1010. Fault is not detected.
z) If S-a-0 fault is at U1-6:
Relay Drop condition:
Port Input is 0101. Fault is not detected.
Relay Pick-up condition :
Port Input is 1100, which is invalid.
aa) If S-a-1 fault is at U1-3:
Relay Drop condition:
Port Input is 0101. Fault is not detected.
Relay Pick-up condition :
Port Input is 1101, which is invalid.
ab) If S-a-1 fault is at U1-6:
Relay Drop condition:
Port Input is 0111, which is invalid.
Relay Pick-up condition :
Port Input is 1010. Fault is not detected.
ac) If R4 is Open:
Relay Drop condition:
LED 1 will not glow. Since LED 2 also does not glow, fault can be usually detected.
Relay Pick-up condition :
Since LED 2 glows, fault cannot be detected.
ad) If R8 is Open:
Relay Drop condition:
Since LED 1 glows, fault cannot be detected.
Relay Pick-up condition :
LED 2 will not glow. Since LED 1 also does not glow, fault can be usually detected .
ae) If LED1 is faulty:
Relay Drop condition:
LED 1 will not glow. Since LED 2 also does not glow, fault can be usually detected.
Relay Pick-up condition :
Since LED 2 glows, fault cannot be detected.
af) If LED2 is faulty:
Relay Drop condition:
Since LED 1 glows, fault cannot be detected.
Relay Pick-up condition :
LED 2 will not glow. Since LED 1 also does not glow, fault can be usually detected.
In summary, Invalid Input is obtained in Relay Drop condition if
OC1 Output is Short
OC2 Input or Output is Open
U1-3 S-a-1
U1-6 S-a-0
Similarly, Invalid Input is obtained in Relay Pick-up condition if
OC1 Input or Output is Open
OC2 Output is Short
U1-3 S-a-0
U1-6 S-a-1
It also shows that a single fault cannot lead to Unsafe Failure.
After the completion of FMECA, we are to perform Fault Tree Analysis (FTA) of the Circuit to identify the causes of Safe and Unsafe Failures .
Fault Tree Analysis (FTA)
We shall now learn how the Circuit can fail to operate. So, we shall make a Fault Tree Analysis as shown below. Fault trees are individually made for both Safe as well as Unsafe Failures.
Safe Failure can occur due to Two simultaneous Faults --- Open Cct Fault in Opto Coupler OC2 as well as Short Cct Fault in Opto Coupler OC1. Unsafe Failure can occur due to Two simultaneous Faults --- Open Cct Fault in Opto Coupler OC1 as well as Short Cct Fault in Opto Coupler OC2.
Unsafe Failure can happen only due to simultaneous Faults in Open cct in Relay NO path and Short cct in Relay NC path.
CCT UNSAFE FAILURE (0101 is read as 1010)
Once we prepare the Fault Tree, we are to calculate the Failure Rate of the Card .
For this, we are initially to find the Basic Failure Rate for each component as per Calculations given in MIL Std 217F . Then we are to Calculate the actual Failure Rate under environmental stress.
The environmental stress factors are :
λ C = Contact Constitution Factor
λ Q = Quality Factor
λ E = Environmental Factor. We consider Ground Fixed
λ T = Temperature Factor
λ s = Electrical Stress Factor
λ R = Resistor Value factor
λ CV = Capacitor Value Factor
Typical Calculations for a Components are showed in Chapter. The following Table shows the individual Component Failure Rates for the Input Interface Cct.
We have seen that Unsafe Failure can be caused only by 2 simultaneous failures – Open Circuit in OC 1 and Short Circuit of OC 2 Output. So, the Failure Rate will be the Product of Individual Failure Rates of the 2 Opto Couplers. From the table above, we find that the Failure Rate of an Opto Coupler is 0.0257 X 10 -6 . So, the Unsafe Failure Rate will be ( 0.0257 X 10 -6 ) 2 = 6.6040 X 10 -16. This is far above the requirement of SIL 4. Total Failure Rate of the Card is 3.375477 X 10 -6 giving a Mean Time Between Failure of 296254.4 Hours.
Reliability after a period of One year (8760 Hrs) i s
R = e – λt
= e - 0.0000033754 X 8760
= e – 0.029568
= 0.97086
Designed Life with 99% Reliability
= (- ln 0.99) / 0.0000033754
= 0.01005033585 / 0.0000033754
= 2977.5 Hrs.
Preventing Common- Mode Failures:
To reduce Common- mode failures, we can feed U1 outputs in two different Ports and that too at different Input Pins . For example, if Pin 3 of U1 is fed to D0 of Port A, Pin 6 of U1 should be fed to D7 of Port B . Then we can match them through Software in Processor Card. See the Diagram of the connection in Fig 4
A Program for the same task is written below in 8085 Assembly language.
START: IN Port 1 : Read Drop Contact Data
MOV D, A : Save Data
IN Port 2 : Read Pick-Up Contact Data
MATCH: PUSH D : Save Drop Contact Data
CMA : Invert Pick-Up Contact Data
MOV B, A : Save Data
MVI C, 00 : Initialize Register ‘C’
LXI H, 0180 : Initialize Registers : ‘H’ and ‘L’
MVI E, 08 : Load Count (8) for the Loop Iterator
LOOP : ANA H : Mask Unconcerned Bits of the Pick-Up* Data
JNZ ONE : If Data Bit is Not 0, Go to Label ONE
MOV A, L : Transfer Data from Register ‘L’ to ‘A’.
CMA : Invert Data of Accumulator
ANA C : Mask Unconcerned Bits
JMP ZERO : Go To Label ZERO .
ONE : MOV A, C : Copy Contents of ‘C’ to Accumulator
ORA L : Make MSB ‘1’ and progress with other bits
ZERO : MOV C, A : Save the new value in Register C
MOV A, H : Load Bit Position Indicator in Accumulator
ADD H : Shift Left for Next Bit indicator
MOV H, A : Save the Bit Indicator Data.
MOV A, L : Load Bit Position in Accumulator
RRC ; Go to Next Bit Position
MOV L, A : Save Updated Bit Position Data
MOV A, B : Read saved Pick-up* Contacts Data
DCR E : Check for completion of all 8 Bits
JNZ LOOP : Repeat Rearranging for all 8 Bits
MOV A, C : Save Rearranged Pick-up* Data
POP D : Restore Saved Drop Contact Data
RET : Go back to Main Programme
CMP D : Compare Drop and Pick-up* Data
JNZ FLT : If not Matched, Go to Fault
This Subroutine is Executed in 34.5 μs, considering that each State in 8085 generally takes 330 ns for execution .
Toggling of Input data to detect Opto Coupler Faults
Instead of keeping the Emitter of Opto Coupler permanently Grounded, we feed ‘0’ from Processor through Port C of Programmable Peripheral Interface 8255, to the Emitters as showed in Fig 5.
After reading the input and analysing, we momentarily feed ‘1’ to the Emitters. Now, all the opto-couplers should be OFF. That means, Opto-coupler outputs should follow the Toggling input from the microprocessor. Now, we again feed ‘0’ from the Processor and analyse the Opto-coupler Outputs finally.
We take inputs from Drop Contacts of 4 different relays along with the Output of U1 Pin 3 of their De-bounce circuits, via Port ‘A’ of 8255. Similarly, the pick-up Contacts of the same four Relays and Output from U1 Pin 6 are taken through Port ‘B’. For 16 relays, Interface to the Card, 4 numbers of 8255 ICs PPI 8255 1 are needed.
The Programme for the Toggle Operation is given below:
MVI A, 92 Programme Four PPIs (8255) having
OUT 03 PORT A -- INPUT
OUT 07 PORT B -- INPUT
OUT 0B PORT C -- OUTPUT
OUT 0F
LXI H, 1100 Starting Location to keep Drop Contacts Data
SUB A Enable Opto Couplers in PPI 1
OUT 02
IN 00 Read Drop contact Status from PPI 1
MOV M, A Save in Location 1100
IN 01 Read Pick-up contact Status from PPI 1
CALL MATCH Rearrange Bits and Match them.
INX H Go to Next Location
SUB A Enable Opto Couplers in PPI 2
OUT 06
IN 04 Read Drop contact Status from PPI 2
MOV M, A Save in Location 1101
IN 05 Read Pick-up contact Status from PPI 2
CALL MATCH Rearrange Bits and Match them.
INX H Go to Next Location
SUB A Enable Opto Couplers in PPI 3
OUT 0A
IN 08 Read Drop contact Status from PPI 3
MOV M, A Save in Location 1102
IN 09 Read Pick-up contact Status from PPI 3
CALL MATCH Rearrange Bits and Match them.
INX H Go to Next Location
SUB A Enable Opto Couplers in PPI 4
OUT 0E
IN 0C Read Drop contact Status from PPI 4
MOV M, A Save in Location 1100
IN 0D Read Pick-up contact Status from PPI 4
CALL MATCH Rearrange Bits and Match them.
STA 1100 Bring Drop Contact Data from Location 1100
LXI H , 1200 Initialize Flag Location for First Relay of PPI 1
IN 00 Read Drop contact Status from PPI 1
CPI 55 Check if this Data is same as the Saved Data
JZ PPI 2
CALL DELAY 1ms Wait for 1 milli second
MVI A, FF Disable Opto Couplers
OUT 02
IN 00
CALL DELAY 1ms Wait for 1 milli second
CPI FF
JNZ FLT If not, Go to Fault
SUB A Again Enable Opto Couplers
OUT 02
IN 00
CPI 55
CNZ PROCESS
PPI 2 : STA 1101 Bring Drop Contact Data from Location 1101
LXI H , 1204 Initialize Flag Location for First Relay of PPI 2
IN 04 Read Drop contact Status from PPI 2
CPI AA
JZ PPI 3
CALL DELAY 1ms Wait for 1 milli second
MVI A, FF Disable Opto Couplers
OUT 06
IN 04 Read Drop contact Status from PPI 2
CALL DELAY 1ms Wait for 1 milli second
CPI FF Check if Opto Couplers are Disabled
JNZ FLT If not, Go to Fault
SUB A Again Enable Opto Couplers
OUT 06
IN 04 Read Drop contact Status from PPI 2
CPI AA
CNZ PROCESS
PPI 3 : STA 1102 Bring Drop Contact Data from Location 1102
LXI H , 1208 Initialize Flag Location for First Relay of PPI 3
IN 08 Read Drop contact Status from PPI 3
CPI 55
JZ PPI 4
CALL DELAY 1ms Wait for 1 milli second
MVI A, FF
OUT 0A
IN 08 Read Drop contact Status from PPI 3
CALL DELAY 1ms Wait for 1 milli second
CPI FF Check if Opto Couplers are Disabled
JNZ FLT If not, Go to Fault
SUB A Again Enable Opto Couplers
OUT 0A
IN 08 Read Drop contact Status from PPI 2
CPI 55
CNZ PROCESS
PPI 4 : STA 1103 Bring Drop Contact Data from Location 1104
LXI H , 120C Initialize Flag Location for First Relay of PPI 4
IN 0C Read Drop contact Status from PPI 4
CPI AA
JZ NXT_CARD
CALL DELAY 1ms Wait for 1 milli second
MVI A, FF Disable Opto Couplers
OUT 0E
IN 0C Read Drop contact Status from PPI 4
CALL DELAY 1ms Wait for 1 milli second
CPI FF Check if Opto Couplers are Disabled
JNZ FLT If not, Go to Fault
SUB A Again Enable Opto Couplers
OUT 0E
IN 0C Read Drop contact Status from PPI 4
CPI AA
CNZ PROCESS
PROCESS : MOV B, A
MVI C, 04
MVI D, 40
CHK : MOV B, A
ANA D
JNZ PICK-UP
MVI A, FF
MOV M, A
PICK-UP : MOV A, D
RLC
MOV D, A
ANA D
JNZ FLT
SUB A
MOV M, A
INX H
MOV A, D
RLC
MOV D, A
DCR C
JNZ CHK
dELAY 1 ms : PUSH D
BACK : LXI B , 0BB1
DCX B
JNZ BACK
POP D
RET
This Programme to check the Status of the Relays Interfaced in One Card takes about 9.64 ms . Thus to check all 16 Input Cards, a total time of 154 ms is needed. It covers up the Transition periods for QN1 Relays.
Card AcCcess
Whenever an Input Card is inserted into one of the I/O Slots, it gets the I/O Slot address , which is Hardwired in Back-plane . Processor Card sends Motherboard Slot addres s through the I/O Bus. When the two addresses match, the Comparator Output Pin gets low and a Decoder and Data Buffer are enabled. The arrangement is showed in Fig 6 .
The FMECA of this sub- circuit is given below.
a) If Pin 19 of Comparator UA is s-a-1:
The Decoder UD and Buffer UC will not be enabled. Processor cannot read ID andID* of Input Card.
b) If pin 19 of Comparator UA is s-a-0:
The fault will not be detected.
c) If any Input Pin of P or Q Comparator of UA is s-a-0 / s-a-1 :
Software data sent by Processor will not match with Hardware data from the Backplane. Failure is same as (a).
d) If any Pull- up resistor for Backplane is Open:
The corresponding Pin may float to either 0 or 1. Depending on that the Hardware and Software addresses may or may not match.
e) If any Input Pin of Buffer UB is s-a-0 or s-a-1.
Software data will be faulty and may or may not match with Hardware data.
f) If Pin 19 of Bus Driver UC is s-a-1:
UC will not be enabled. Card ID and ID* cannot be read.
g) f) If Pin 19 of Bus Driver UC is s-a-0 :
UC will be permanently enabled and the fault will remain undetected while accessing the Card.
h) If Pin 1of Bus Driver UC is s-a-1 :
UC will not allow reading Card ID and ID*.
i) if any Data Pin of Bus Driver UC is s-a-0 or s-a-1 :
Processor will get wrong ID and ID*.
j) If Pins 1 and 19 of Buffers UE or UF are s-a-1 :
Processor will not get ID or ID* depending on whether UE or UF has got the fault.
j) If Pins 1 and 19 of Buffers UE or UF are s-a-0 :
UE or UF will be permanently enabled . There will be Bus contention while reading ID and ID*.
