IIRC, yes.
AFAIK, the resemblance to Intel hex is not coincidental: Modicon were using 8080's at that time.
GE / Fanuc decided that the hex transfer is losing channel capacity and designed the binary encapsulation for their terminals (RTU) with all the flaws built-in.
Cannot comment about PPP-HDLC encapsulsation, but if you look at the original HDLS specs they contain sync characters and address fields as well as CRC, which was usually done in a dedicated hardware controller. Having worked on HDLC[1] controllers that were boxes added to PDP-11s, long before dedicated chips came out that were HDLC controller on a VLSI chip.
HDLC was the backbone of many protocols like X25 and was one of the granddaddies of packet based protocols.
[1] if you see mention of SDLC this was IBM version of HDLC that was slightly different but the difference was minor so even early VLSI chips had one bit to determine which of HDLC and SDLC it was to use.
The PPP-HDLC encapsulation substitutes the bit stuffing in the bit-synchronous protocol with byte-stuffing, as it's pretty obvious the the bit-stuffing gets too complicated with an octet-only transfer (remember that HDLC/SDLC is a bit-level transfer protocol).
For details, see RFC 1662, e.g. .
Ah. I had never realized that Modbus-ASCII and Modbus-RTU came out of two different companies.
Been there, done that. It sucked.
Fax machines use HDLC with bit-stuffing, but the modem chipset I was working with transferred data in octets.
I would also recommend CAN. Microchip makes a nice SPI to CAN controller which is easy to connect to most MCUs. Even if you have to bit bang the SPI interface, it does not require too much MPU resources. In stead of optical isolation ,I would recommend the new generation GMR isolation devices from NVE, analog devices and others. A single 16 pin IC is available with 3 out, and one input channel which is ideal for isolating at the SPI interface level. These devices can handle data rates of up to 50 Mbits/s.
Regards Anton Erasmus
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