No display but audio works

[willnotwish]

The radio connects to the car via a spade terminal ground connector, an aerial coax connector, a round multi-pole connector for a CD changer, a rectangular multi-pole connector for power and speakers and a second rectangular 14-pin multi-pole connector (plug and socket pair).

The latter connects to a separate display unit, which has two rows of characters. The upper row displays the time of day, fuel consumption (mpg/range), temperature etc. The lower row contains audio information such as the radio station, tuning frequency, CD track etc.

It is the second (bottom) row which is not working.

To cycle through the top row options, the driver presses the Display button on the radio. This doesn't work at present.

My goal is to get the display unit working with the replacement radio I have just bought.

The radio itself has no display. It appears to function OK in terms of audio, but it is difficult to use (e.g., station tuning) because there is no display. The station scan feature (setup) doesn't seem to work.

I know the display is OK because it worked properly with the previous (original) radio. The problem was then that the radio itself didn't work. It wouldn't switch on and off reliably. When it did eventually switch on, no audio could be heard. But the display worked.

[willnotwish]

I need to get the display working so I can see what the radio is doing.

[willnotwish]

The display has a local "Reset" button but I think all that does is set the time. The only way to toggle between the display options on the top row is via the Display button on the radio. This has nothing to do with the audio.

The radio functions not only as a radio/CD player, but also as a controller for the display.

[willnotwish]

I would expect there to be data flowing between the radio and the display. There could be a CAN bus between the two. There are six individual wires in the connector. Two are shorted to earth inside the radio.

If there is indeed a CAN bus present, I would expect to be able to measure the CAN termination resistor (120Ω) between the CANH and CANL pins at both the plug and socket (with the radio disconnected).

[willnotwish]

I can't measure any resistance close to 120Ω between any two pins at the radio connector.

Of the six wires, two are grounded. Three more go directly to the CD changer socket. Presumably the remaining pin is for comms between the radio and the display.

Maybe it is just one-way traffic: from the radio to the display.

[willnotwish]

In addition, there are no twisted pairs anywhere in sight: a further indication that a CAN bus is not in use.

[willnotwish]

I guess the next step is to monitor the data pin (as I am calling it) using a scope. For this I need to break out the connection from the radio to the display so I can attach a scope probe.

[willnotwish]

As an aside, here's a nice simple article about CAN.

[willnotwish]

It could be that the physical connection is OK, but the new and the original radios are different somehow. They have slightly different part numbers.

It could be a software incompatibility. The software loaded on the new radio might be incompatible with the display.

First of all, let's see if I can see any data on the line.

[willnotwish]

As expected I saw nothing on any line with the new radio connected. But there are signals present with the old radio (audio doesn't work, but communicates OK once powered up) after a few switch-on attempts.

There appear to be two data lines: the red and white wires.

The red looks like some sort of chip select. After it goes high, the white signal looks like it's transmitting data in a serial fashion. I haven't attempted to decode it to see if it's sending 8-bit bytes, but that is plausible. When all the data has been sent, red (CS) goes low again. After 200ms or so (I'll need to take some measurements), the cycle repeats.

I did manage to get some data into the display unit before the new radio stopped transmitting. Whatever is sent remains on the display until it is overwritten.

[willnotwish]

With the new radio connected, the red line is stuck at 2.5V or so. No data is sent on white.

[willnotwish]

Physically, the output stages of the red and white lines look similar. I need to reverse engineer the board.

[willnotwish]

To make it easier to debug the hardware, it would be better if I could power the radio on the bench. The old unit -- once powered up -- searches for stations continuously with no success. As a result it keeps updating the display to show a changing frequency as it scans the FM band.

[willnotwish]

A further observation is that the automatic tuning doesn't work on the new unit. On the old unit, pressing and holding the RH button causes the unit to beep and then search. No beep is heard on the new unit.

[willnotwish]

Markings on three-terminal SOT-23 packages in the output stage (some sort of buffer, I guess):

• Z_S (Q770)
• BS (Q771)
• 94 (Q772)

94

DTA114T PNP with built-in bias resistor

[willnotwish]

Z_S

(maybe)

2PC4081S General purpose NPN

[willnotwish]

BS

Could be 2SC5658 or similar NPN small signal amplifier 50V 150mA

[willnotwish]

I think the output driver is some sort of open collector arrangement, but it's difficult to be sure.

[willnotwish]

I'm going to try powering up the board on the bench. I have a really old board purchased as a spare some years ago when I looked at this issue before. I have "modified" it extensively. It may not run. Let's see.

[willnotwish]

The new radio switches on fine when you press the on-off button, and that you can change the volume, switch tracks and so on. Given that, is it reasonable to assume that the main microprocessor IC801 is powered and working OK?

I think so.

It's hard to believe that the unit would work at all if no.

Markings on the CPU:

NEC 780078 GK
531
0416 EP001

0416 is a date code: 2004, week 16

Datasheet download.

[willnotwish]

Front panel controls are scanned/sampled by IC801 (the microprocessor). I think it must be working in order even to turn on.

[willnotwish]

The microprocessor appears to be sending data in bursts to the display unit. At least, that is how it's supposed to work.

This happens on the original unit, but is not working on its replacement.

Why?

If the microprocessor's output pins are undamaged electrically internally and being driven correctly by on-board software (IC801 has on-board ROM), then the problem may be with the driver circuit.

[willnotwish]

Let's name the units A, B and C.

A is the original factory-fitted unit from 2005. B is a recently bought replacement from 2009. C is an incomplete scrap board purchased some years ago.

A: no audio, very unreliable switch-on, talks to display OK.

B: audio works fine, switches on easily, no display.

C: a bare board. Really only useful for spares.

[willnotwish]

I need to investigate how the display driver circuitry works. There appear to be three outputs with similar (two look identical and one differs slightly) designs.

A possibility is that the third output outputs data to control the remote display options when the Display button is pressed on the radio.

It seems unlikely that all three drivers have failed.

[willnotwish]

Another idea is to compare the PCB layouts of boards A & B to look for differences in the display driver circuitry. It could be that this changed in the intervening four years.

OTOH, given that the software is contained in the integral CPU ROM, it is quite plausible that A & B could be physically identical but running on different software, updated as bugs are found in use.

[willnotwish]

I just captured another trace on the CS and DATA lines (as I am now calling red & white).

There is communication between the radio and the display even when the radio is "off" (powered by permanent 12V and ignition ON).

Looks like the CS (yellow trace) is first driven low. Some 6.5ms later 12 bytes (maybe) of data is sent at about 0.8ms intervals.

[willnotwish]

Let's see if I can power up unit A on the bench and repeat this test.