Adding rs232 to a meter? First time trying to reverse engineer. - Page 1
Forum section: electronics / general

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 09, 2014, 01:34:38 am
content: So I cracked my meter open the other day to calibrate it, then I remembered an article I saw on hack-a-day about adding rs232 to a cheap 4 dollar Chinese meter and thought Id like to do that to my meter. The guy used a PIC and just intercepted the data that goes to the chip. But he doesn't really explain how he found the the pin he scraping data from, or how the data is formatted.http://www.instructables.com/id/DMM-Piggyback-add-RS-232/This is not my best option but it may be my only option, can anyone explain to me exactly what hes monitoring? Did i miss something or is this very vague?I have a decent meter a Klein MM2000, It's a decent meter in the the 100 dollar range, with RMS, bar graph etc. So I figured it may have a nice enough chip that it has rs232 built in, just disabled. There are apparently quite a few meters out there like this. So I did some digging around and found out it has pins for TX and RX, but im not sure they do anything, I.E. there not programmed to do anything in the firmware. Both RS and TX measure at Logic Level HI with no data moving in or out of them when I switch meter functions. The meter uses a Fortune SemiConductor FS98024 DMM chip,http://www.ic-fortune.com/upload/Download/FS98O24-DS-13_EN.pdfThe section on the serial port and GPIO doesnt talk about anything except the register structure, and Im pretty sure Im not going to be able to program the chip, I cant find any information on programming an FS chip except this documenthttp://www.ic-fortune.com/upload/Download/FS982x-IDE-UM-10_EN.pdfIm pretty sure it applys to this chip, but its not very helpful.Im hoping some reverse engineering guru or expert working with obscure chips can give me some tips.This is about where ive gotten, any ideas to get data logging on this meter would be appreciated.

author: linux-works
author_rank: Super Contributor
timestamp: May 09, 2014, 02:16:48 am
content: the vichy vc99 I have supports serial data logging and I just had to tap into 1 or 2 pins, convert ttl to rs232 (or usb) and that was it.do you see any rs232 icon or label in the lcd display? if the meter CAN do it, there would be an indicator on the display. and a button to enable it (the button might be hidden below the plastic front bezel).I'm not sure any meter always outputs data; usually there is a button to turn it on and off. try to find that, while sniffing the tx/rx lines.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 09, 2014, 02:46:52 am
content: No Icons and no rs232 enable pins as on other meters, just a GPIO port with TX and RX. Im thinking in order for it to do data logging it would have to be written in the firmware.Im a bit confused though because the data data sheet says the chip has EEPROM and SRAM bit then under special chip features it says the chip is OTP rom, one time programmable.

author: Harvs
author_rank: Super Contributor
timestamp: May 09, 2014, 04:17:03 am
content: Quote from: rwgast_lowlevellogicdesin on May 09, 2014, 02:46:52 amThe datasheet says it has EPROM (note the single E for Electronically Programmable ROM, as opposed to the double EE having Erasable in it.) That makes it OTP.

author: macboy
author_rank: Super Contributor
timestamp: May 15, 2014, 02:31:47 pm
content: I looked at the instructable link you gave.First, that design does not somehow intercept data from the DMM, but instead, it uses the PIC ADC to read the 0 to 200 mV input voltage that the DMM is going to also digitize to get its display. The PIC then sends the result of its own ADC to the computer over RS-232. This is not what you want.Second, that design is inherentlyextremely unsafe, as there is no isolation between the meter and the computer. The computer's ground will be connected to (I assume) the meter's common lead. So the meter is no longer floating, but is ground referenced. Don't. Just don't.All "real" external data interfaces use optical isolation. Sometimes optocouplers are used internally, other times, the actual connection to the meter is using a "cable" that actually has IR Tx/Rx, but no electrical connections.Quote from: Harvs on May 09, 2014, 04:17:03 amEPROM = Erasable Programmable ROM. If the microcontroller die is in an expensive "windowed" package, then it can be erased by exposure to UV light. The exact same die in a black epoxy package is effectively OTP, since it can't be exposed to UV to be erased. This used to be very common; you would use one or more of the expensive erasable versions for development, and use the OTP one (which is always the exact same die) for the final product. These days, FLASH program memory is almost universal.EEPROM =ElectricallyErasable Programmable ROM.