Ouch my eye!

Today I took on the front-panel control/display board. The board is pretty much charred as you can see. It has quite a few traces on it, but is fairly simple in function, thus making it relatively easy to decode.

Before even doing any cleaning I tried  to make some pretty good guesses as to the architecture, the next paragraph is my assessment from before I started.

There are 2 IC’s on it, which are likely some sort of tri-stating buffer, allowing the switch signals to be multiplexed onto a data bus. The data bus also feeds to the LCD.  Then there are the 2 LED’s, these are fairly large and probably consume a bit of current, which would explain the two transistors beneath them, likely acting as low side switches. There’s a 3rd transistor on the board, which I’m not sure of the function yet, but it should become more obvious as I trace it out.

Now lets get into it and see how well my guess did.

First I cleaned the board as best I could to expose as much of the circuit as possible. The PCB substrate is charred, and delaminating in places, so I had to be careful in certain areas [mainly the top edge]. Once cleaned, I made note of the various components to come up with a rough Bill of Materials for the board.

Bill of Materials: [some of the designators are guessed]
Resistors: [Values measured where possible]

R1, R2: 4K7 - Base Res for Q1/Q2
R3: SIP 100Rx9 - Current limiting resistors for D1/D2 arrays
R4: SIP 10Kx9 - Pull-ups for keypad
R5: SIP 10Kx7 - Pull-ups for keypad
R6: 10R - Current limiting resistor for LCD backlight
R7: 10K VR - for LCD contrast [VEE]
R8: ??? - High side offset for Contrast [destroyed by fire]
R9: 4K7 - Part of RC with C3
R10: 1K - Base Res for Q3

Capacitors: [values guessed]

C1, C2:  0.1uF - Bypass
C3: 0.1uF - Part of RC with R9
C4: 0.1uF - Bypass for VEE

Diodes/LED’s:

D1: MU02-2201  60mA Red LED [array of 4 LED's]
D2: MU02-5201  60mA Green LED  [array of 4 LED's]

Transistors:

Q1, Q2: 2n2222A - NPN - Low side switch for D1/D2
Q3: MPSA56 - PNP - VCC Switch

IC’s:

U1, U2: MC74HCT245 - Octal 3 state bi-directional buffer

Connectors:

J1: 1x16 LCD
J2: 1x16 Keypad
J3: 2x13 Main Connector
J4: 1x2 LCD Backlight

Unknown:

SIP for LCD data lines, may be RC or other termination network

Going on, here is what I knew about the connector pinouts

J1 Pinout: [well known/documented pinout for dot-matrix character LCD’s]

1: GND
2: VCC
3: VEE
4: RS
5: RW
6: EN
7: D0
8: D1
9: D2
10: D3
11: D4
12: D5
13: D6
14: D7
15: Backlight +
16: Backlight - (Gnd)

J2 Pinout: [established in part 1 yesterday]

1: Common
2: Start
3: Menu Up
4: Select
5: Menu Down
6: Escape
7: Z
8: Pause
9: Next File
10: Resume
11: Right Arrow
12: Down Arrow
13: Left Arrow
14: Prev File
15: Up Arrow
16: X-Y

J4 Pinout: [alternate backlight connector, well known/documented]

1: Backlight +
2: Backlight - (Gnd)

Also from the BOM, we know the transistor and IC pinouts:

The board is laid out very orthogonal, with horizontal traces on the top side, and vertical traces on the bottom side, which makes tracing easier. So now it is just a simple matter of tracing out the connections with a continuity tester.

The Power Path:
Using the IC’s, I determined that GND is indeed the copper pour on bottom, and top of the PCB, VCC is run as a thick trace [meaning the keypad common is GND]. Q3 sits between VCC for the board, and the V-input of the connector [pins 2,25,26], likely acting as a high side switch to turn the front panel on & off. Strangely the resistor networks for the keypad are on the main Vin, and not on VCC, this may be a small error in the design, though likely would not cause any problems. The only other components on VIN are the two bright LED’s, and this is where I would expect to see them. As stated GND is a copper pour over the entire PCB, and is connected to pins 1,15 & 24 of J3.

The Data Path:
The data lines D0-D7 appear, in order, on pins 7-14 of J3. They are also mapped to the A side of both the 74HCT245 buffers, nicely to the A1-A8 pins respectively. The Direction pin [pin 1] of the buffers is tied to ground, meaning they are fixed in direction with the B side as input, and the A side as output. The data lines also run to D0-D7 of the LCD connector as anticipated. The data lines are also terminated by a SIP device at the LCD.

The LCD:
Beyond the data lines discussed above, the LCD has 3 control lines RS, RW, and EN, these map to pins 4 thru 6 of J2 respectively. The LCD also has a backlight which is always on, and fed via a current limiting resistor R6. There is also a contrast adjustment variable resistor (R7) mounted on the bottom side of the board connected to the VEE pin of the LCD.

The LED’s:
The two large LED’s are actually arrays of 4LED’s each, and are rated at 60mA. The each of the LED anodes runs to one of the resistors of the R3 SIP resistor network. The cathodes of each colour are tied together and route to Q1 or Q2, which when enabled connects them to ground. The base lines for Q1 and Q2 route to pins 18 & 21 of J2.

The Keypad:
The keypad lines are all pulled to VIN via the two resistor networks (R4 & R5). [not sure why this isn’t VCC] When U1 or U2 have their outputs enabled, the switches will be mapped directly to the D0-D7 lines. The Output Enable (OE) for U1 runs to pin 3 of J3, and the OE for U2 runs to pin 16 of J3.

The keypad mapping beeps out as follows:

U1 B1[D0] J2. 8 - Pause
U1 B2[D1] J2.10 - Resume
U1 B3[D2] J2.2  - Start
U1 B4[D3] J2.9  - Next File
U1 B5[D4] J2.6  - Escape
U1 B6[D5] J2.4  - Select
U1 B7[D6] J2.3  - Menu Up
U1 B8[D7] J2.5  - Menu Down

U2 B1[D0] J2.13 - Left Arrow
U2 B2[D1] J2.12 - Down Arrow
U2 B3[D2] J2.16 - X-Y
U2 B4[D3] J2.14 - Prev File
U2 B5[D4] J2.11 - Right Arrow
U2 B6[D5] J2.15 - Up Arrow
U2 B7[D6] J2.7  - Z

The Surprises:
As mentioned in the power path, there is Q3 that sits between the VIN of J2, and the VCC network for all the components on the board. The base of this transistor is controlled via pin 23. Not sure why they wanted/needed this controlled, but there it is. The second surprise was an RC network that connects to pins 19 & 20, as well as pin 11 of U2 [B8->D7]. I’m not sure what the purpose of this network is in this application. It may be that it is debouncing some other input from the wiring harness, and passing it onto the data bus.

So that gives us our final pinout for J3 as:

 1 GND            2 VIN
 3 OE U1          4 LCD RS
 5 LCD RW         6 LCD En
 7 D0             8 D1
 9 D2            10 D3
11 D4            12 D5
13 D6            14 D7
15 GND           16 OE U2
17 N/C           18 Green LED
19 RC [U2.B8]    20 RC [U2.B8]
21 Red LED       22 N/C
23 VCC Enable    24 GND
25 VIN           26 VIN

All-in-all I did pretty well with my initial guesses, although it was not hard to guess, based on the simple functionality. With the above information, I now have enough to make my own compatible/replacement that will work with the original electronics. This should hold me over until I can get my own custom controller up & running.