In-house design decode & repair of PCB-295 cards
1. Introduction
This project is about repairing a card which is called the PCB 295. PCB 295 is used in the driller console for displaying various drilling parameters. The driller console is placed at the derrick floor of the rig.
Drillers use an instrumentation system which is called the Advanced Instrumentation System or AIS. AIS has many sub-assemblies like the driller console which is in the derrick floor and the DAQ (Data Acquisition Unit) which is in the AIS bunk house.
The DAQ is connected to the driller console by means of a serial link. The driller console has a series of horizontal and vertical LCD displays. Each of these horizontal and vertical displays require a display controller. This display controller is called PCB 295.
The values which are sent by the DAQ from the AIS bunk house are conveyed via redundant serial link by means of a stream which is a packet containing (address, data) pairs. All the parameters which are to be displayed are conveyed one by one serially. Each PCB 295 card has DIP switches which can set the address of the card.
Depending on the address received in the serial link , the card accepts the packet destined for it and drops the remaining. Once the packet is decoded, it is displayed in the respective horizontal or vertical display. Drilling parameters like hook load, bit weight, strokes per minute are displayed in the respective display(s).
The hardware design of the card has been understood and so far 18 cards have been repaired. These cards had failed over a period of time. The cost of each such card is about Rs 66,615/- each. Therefore, the total cost of repaired cards is about Rs Twelve Lakhs . Moreover, these cards are already out of production from OEM and these spares will be of help to meet contingency requirement till DAQ is upgraded to the next OEM supported version.
2. Important part list:-
U1-CD 4011 BF (NAND gates) (CD 4011B)
U2-Intel 80c31BH (microcontroller)
U3-SN74HC373N (D Latch)
U4-AS65-6264 (RAM)- available RS INR 369 each
U5- ST M27C256 EEPROM
U6-CD4049 (Hex buffer/converter)
U7-HCF4047B (monostable multivibrator used as Watchdog timer)
VR1- LP2950 ACZ-5.0 voltage regulator 3 terminal
Q1 Q2 - 2N2907 NPN transistor
3. Tools required:-
1. Oscilloscope 20 MHz or Higher
2. Multimeter & LCR meter
3. 25 W soldering Iron
4. PTH Desoldering station
5. Assorted Tweezers, snips, precision screw driver set.
6. Mobile phone with 5X optical zoom
4. Key Learnings:-
1. Troubleshooting microcontroller based circuit cards.
2. How watch dog timers are used to reboot microcontrollers in case of card errors
3. Desoldering of PTH ICs using vacuum desoldering station and to remove ICs from board without damaging PCB traces
4. How two serial signals are selected by microcontroller for maintaining redundancy.
5. How trigger circuits are designed for making a re-triggerable monostable timer circuit.
Download the Article to view the design, troubleshooting and repair exercise of these cards.