I began using the HP 9826/9836/9845 and HP 9000, series 300 controllers back in 1980. Those were the instrument controllers of choice for Aerospace and military test stations designed around the HPIB (IEEE-488) and Rocky Mountain Basic (RMB). As personal computers matured in the late 80’s, HT Basic and generic HPIB card’s phased out those expensive HP controllers without the need to port the RMB software. This was a major advantage at the time. In the early 90’s, NI LabView became the new PC based software standard for instrumentation control and data acquisition. For your application, the least expensive port would be to use Python. It has native HPIB and Modbus drivers all for free and fully open source supported. With a minimal PC setup and USB HPIB interface ($100/eBay), you probably could port the RMB software over with a minimal effort gaining a much longer system shelf life.
For the first 20-years of my career, I designed and programmed ATE test stations for military and aerospace. So, I am very familiar with HP RMB, HT Basic, NI LabView and IEEE-488 I/O. The last 10-years of my career has been with a major water utility integrating instrumentation using proprietary SCADA software for control and data acquisition. Much of this instrumentation uses Modbus over RS-485 & IP.
Given your situation, the fastest way across the finish line would be to use the existing HT Basic controller setup and connecting the analog signal out from the ABB EL3020 to the A/D input of the HP 34970A. This eliminates your Modbus programming issue. With regards to the instrument signal noise/accuracy issue: 1) Set the full scale (F.S.) output of the analog signal to the lowest maximum measurement value needed. For example, if the instrument measurement F.S. is 10 “units” but you only need to measure 1 unit max for your application then scale the analog signal output F.S. for 1 measurement unit max. 2) Take an average of multiple readings that best represent the desired instrument display and/or digital reading. 3) You can also introduce an RC filter network across the output signal to further attenuate any signal noise.
With regards to capturing the serial Modbus data and importing it into HT Basic. Unless your job is dependent on you successfully implementing that approach – I would abandon it.
I hope this helps.