From hardware viewpoint device would operate in two modes – first as serial console to DM3058 multimeter and then as USB mass storage device when connected to PC. Early on I decided to use Texas Instruments ® MSP430F55xx micro controller for this task as it has stable USB support in hardware and I had some experience playing around with USB devices on MSP430F5529 LaunchPad board. For data storage I would use SPI flash chip, then I would need RS232 transceiver and some input protection ICs. I wanted to use lowest spec devices still adequate for the task. Parts would be SMD but still suitable for hand soldering, except for switches and alike that would be though hole, at least for first prototype. I ended up choosing MSP430F5510 micro controller for which I could use nicely designed Olimex ® MSP430-T5510 development board and set up rest of schematic on a bread board. For data storage immediate choice was to use somewhat outdated but still available Atmel ® AT45DB161 16-megabit SPI flash IC. I would see where it all will take me. Renesas ® ICL3221 would serve as RS-232 transceiver for connecting to DM3058E serial port. It has single RX/TX pair instead of classic MAX232 dual driver and receiver arrangement. It is also a bit cheaper than other similar ICs and ICL3221E version has somewhat better ESD protection specs on RS232 pins.

Quite a lot of wiring. MSP430-T5510 development board can be seen in red and RS232 transceiver on breakout board. SPI data flash and EEPROM ics are hanging out on wires.

To clear any doubts I would not consider device I am working on as a hack of any sort. Serial port specification and data exchange protocol for DM3058E is readily available at Rigol ® website. I am also not affiliated with the company in any way except that I have bought DM3058E multimeter through their product clearance web site.