Got to love R2 in the power supply!
Well yes, kinda stands out eh ?! I don't know why they needed to make that shunt wire sooooo long. Who knows. To help cooling/accuracy maybe.. though accuracy is hardly a concern in the design of this particular PSU for sure. So don't know.
Anyway how, I have just validated my ordered at Farnell, to get the bits I need to finally put the Metrix back in one piece. However sadly, the order will be delayed because of the 28 pin LCD header connector, as it comes direct from the manufacturer's stock. They state a delay of 5 working days, so at best, if I am lucky, I will get the parts on Saturday morning. So in the meantime, I will work on trouble-shooting and hopefully fixing the PSU.
Since this topic is about tearing down the Metrix, I guess the schematics I uploaded are in line with this topic and that deserve some comments as part of the tear down, no ?
I see that dozen or so people have downloaded it ! So it must be of some interest to some people, more than the few of us who actually post in this topic anyway.
So... what could we say about this board/schematics ? I spent a few hours reading the m again and again...
Overall, it's more complex/convoluted that I thought it ought to be ! To start with, the representation of the contacts/tracks of the main rotary switch, is not so easy to understand... one can eventually figure out the broad lines, enough to get a sufficient understanding of where the signals come and go... most of them anyway, but saying that I understand easy and every little detail of it, would be rather presumptuous...
Anyway. The main thing that I find interesting, is the overall architecture of the thing. You have got the big "DMM5" proprietary chip on the main board, and the little LCD board with a big chip on it, and a header connector linking the two. So, one would assume that it's all very classic an arrangement : the DMM 5 chip has got to be the main processor doing it all, and it just talks to the LCD board to control the display, and the chip on the LCD board is obviously the LCD driver. Right ? No, totally WRONG ! LOL
Looking at the schematics, one quickly sees that the big DMM5 chip handles only the analog stuff, doing most of the magic. The main board actually is 100% analog. The LCD board is actually the main digital board, everything digital is all happening in there ! The LCD driver is actually not the driver, it's the main processor/controller, a modest 4 bit MCU from NEC, featuring 16KB of PROM (in my meter anyway, a 316 version, not the 308 / 8KB version pictured in the schematics), 512 bytes of RAM, and running at around 4MHz.
So, it's completely upside down compared to most other meters I would guess. That little LCD board contains everything digital, and the main board and that DMM5 proprietary chip, are completely dedicated to handling all the analog stuff. So, a very clear separation between analog and digital. I guess this helps achieve the high accuracy of this meter, by keeping away the fast digital transients, from "polluting" weak but precious analog signals. Well, just a supposition... after all, there are many meters in the MX series, hence sharing this same basic architecture, yet most of them are hardly precision instruments. But maybe Metrix being a fairly small company after all, couldn't afford to design 20 different architectures for each and every of their instruments, so they invested their R&D money in something that would be suited to the most accurate of their meters, then all the less accurate ones would therefore also benefit from it.
The LCD / digital board
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As we have just seen, it's actually not just the LCD board, but the main CPU board. Besides the LCD controller, the MCU doesn't integrate very much in terms of peripherals, just the bare minimum to be able to operate the meter : a couple 8 bit timers, a serial interface so it can handle the external Cal EEPROM but strangely it is not being used, the pins are used as general purpose pins, so chatting with the cal EEPROM has got to be implemented in S/W... why.
.. and a few digital I/Os to handle the membrane keypad, RS232 IR link to access the outside world, and a few more input lines to get the status of the Fuses and position of the rotary switch (using 4 lines, " ROT0 " up to " ROT3 ".
And of course, the data bus and usual associated control lines, so it can control the DM5 chip to get back the readings from the DMM5 built-in ADC, and also drive the 3 and only digital lines featured by the DMM5 chips (labeled "AUX 1/2/3" ) in order to select ranges and enable or disable the 10M input resistor on the mV range, according to what the user is asking for in this regard.
The MCU actually does not feature an UART (the serial interface is just for simple SPI links and the like). The TX and RX signal are then tied to general purpose I/O pins. The link must then be implemented in software, which might be why, I would guess, the CPU runs at a rather fast almost 5 MHz, which would otherwise be unnecessary I think (?), to operate this meter. At any rate the DMM5, which is connected to the data bus of the MCU primarily to send back the results of its internal ADC, runs from a modest 32+ KHz watch crystal !
The LCD board also includes the backlight circuitry, for those meters who do have it (not my vanilla MX55 anyway). It's totally independent from the MCU, so should the MCU fail, you would still be able to turn light on and off. It uses a TLC393 comparator (comparator of choice throughout this meter) to H/W debounce the membrane button, then goes to a couple D flip-flops (usual 4000 series CMOS) to perform the toggling action. The LED backlight is then driven by a discrete transistor.
Check Fuses : once again, a clear distinction between the analog part of the detection circuitry, which is on the main board, and the digital part of it. So, the digital board gets the analog feed from the main board, and adds a little bot of shaping circuitry so it can apply it to the digital IO pins of the MCU. In the case at hand, a couple comparators are sued for FUSE1 (TLC 393 as always), and a third being used for FUSE2.
The Cal EEPROM : an ST 93C46 ( 461 actually on my meter). NOthing special you say ? One point worth noting though : they are not rated at the usual 10 year data retention period, but a surprisingly high, and much more suited to the task... 40 YEARS ! And the C461 I actually have in my meter, is rated at 100 years ?! Dear... didn't know such EEPROMs even existed ! So I found that interesting.
Size-wise, it hold only 128 bytes, so just enough to cram in the required cal data.
The main / analog board
____________________
So, that big DMM5 chip, to start with. It's main purpose is then to assist in implementing all the analog magic required to put the meter together. It embeds teh ADC and sends its readings to the digital board via the CPU(s 4 bit data bus).
It houses the buzzer functionality, said buzzer is connected straight to it. Might be why it is scratchy/not latched... the chip being so analogy in its design, I guess a latch was not in order.
It features a current output which goes straight to the input jack. Of course this is used when in ohms mode, or capacitance or diode test.
In order to produce accurate(ish) current for the various required ranges, it uses an external 3 pins 1.25 voltage reference, along with a little cascade of carefully calculated resistors.
It has a digital input dedicated to measuring frequency. Being digital, the signal passes first through some external shaping circuitry, in the form of a couple comparators, yes, TLC 393 again...
The precision/potted divider network is connected straight to the DMM5 pins.... along with no less than 12 capacitors ! Boy what the hell do they all do. The 3 trimmer caps found on the board, are actually all part of this bunch.
There are quite a few analog inputs and output on this DMM5, sometimes a bit hard to follow. Signals come in, then out, then in again via some other door... its a bit hard to follow I admit. From what I can gather : two signal come from the "front-end" of the meter/ input circuitry : "PR" and "10M". 10M as its name suggests, goes to the highest/10M tap of the precision network. PR attacks it from the 1M step just below.
From that, the DMM5 generates several other signals : "AN0" , which is then sent to the shaping circuitry which produces a clean digital signal which is fed back to the frequency input of the DMM5.
The DMM5 also output a couple more signals : AC1 and AC2. These feed a couple things : the RMS converter, and a "discrete"/improvised 10 bit DAC made out of a binary counter (a 74HC4040), coupled with a 10 stage/bits R-2R ladder, as you do... but whose goal escapes me, any idea ? These two signals are then fed to an analog switch (a classic CMOS 4052 chip), whose output, labeled AC3.. is then fed back to the DMM5 ! See ? signals just keep going in and out of this DMM5 chip ! LOL
This analog switch is actually a double switch, and serves a second purpose : switching R25 on and off, the 10M input resistor, in order to be able to obtain a super high impedance mode when using the mV range.
The DMM5 has 3 main analog inputs : IN1 to IN3. IN1 is the one used to read DC voltages, through the precision divider network. IN2 is the input used to measure currents. IN3 is used I think when measuring ohms or capacitance or testing diodes : the DMM5 outputs the current to the jack/into the DUT, then reads back the resulting voltage via this IN3 path. In the path of this signal, is a complex network of no less than 12 resistor... which really make up for a simple two resistor voltage divider, as you can see. But I guess it was necessary to put several resistors in series to get the high voltage protection a single tiny SMD resistor had no chance to meet. A similar arrangement is used nearby, by another circuitry in the front end : a chain of 5 resistors to make up for one high-voltage item. said resistors drive a rather complex arrangement of 5 bipolar transistors.. though their final goal is clearer : they drive a FET that shunts the input jack. Probably used to discharge capacitors once they have been tested ? Something like that...
As for the section related to current measurement, it has been already described in a previous post. Basically the signal goes through a ladder of shunts, and an analog switch taps it where necessary, for the required range, then the signal is sent straight to the "IN3" in put of the DMM5, without any further processing. Switch control inputs are driven directly by AUX 1 and 2, 2 of the 3 digital I/Os the DMM5 features. Obviously, again, the DMM5 doesn't even know what it's doing... it merely drives this pins blindly, ordered by the MCU sitting on the LCD board. The DMM5 merely passes on the orders...
The current ranges are switch by an analog switch bu sadly it a proprietary Metrix-ITT switch, so not data sheet available. but well, its operation doesn't seem to hold much mystery. The terminals being labeled on the schematics, and being rather self explanatory. Just a switch. 4 inputs controlled by two digital lines, "AUX"' 1 and 2, which come directly from the DMM5 chip, and which are remotely controlled by the CPU on the LCD board via the data bus.
Well that's all I can think of. At any rate that give me a good enough understanding of these MX50 series meters, so I feel confident to buy some more, as I know how they work and could potentially fix them is need be.
So that is it, the tear down is now complete I think !
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Topic: Metrix MX56C / BK Precision 5390 Multimeter Teardown (Read 41319 times)