Is it possible through Digi One IAP over Modbus TCP/IP to access DF1 network of HMI & PLC (SL5/04) without configuring both devices but configuring Digi One IAP?
Yes - with limitations. It needs to be DF1 Full-Duplex or Radio-Modem. Forget HD/Half-Duplex.
The SL5 PLC has tables of data, which is often 16-bit oriented (like an N-file is say 256 16-bit words). In summary, the DOIAP allows you to map 250 of those words to Modbus Holding registers. It is not as fancy as some competing products, but it usually gets the job done. It CANNOT do things like special 16-to-32 mapping. In all cases, if it does what you need - great. If not (like you need F files mapped to Enron floats), then you’ll need to buy a different product.
Look at this app note: http://ftp1.digi.com/support/documentation/90000647_B.pdf