Coils and registers in Modbus

Coils are digital status so (0 and 1) , holding register are analog values wich can be read or write it using the function code 3 or 6 depend if you want to write 1 single register or multiple registers.
I saw this terminology first time when working in Honeywell PlantScape / Experion QuickBuilder interfacing to FSC Controller...

Do you know why do they use such an obscure terminology? Coils?? Registers??? instead of simply stating Digital and Analog points.
My old boss used to work at modicon. I asked about that once. They are there for historical reasons. When they first came out with the PLC using modbus, the idea was to hide the fact that you are using a computer. I think it's kind of silly myself.


Lynn A Linse

Yes, but do remember the first Modicon "PLC" was introduced by the 1970s as a replacement for relay boards - long before Apple, IBM PC, or even the idea of a "personal computer".

Consider how happily the average engineer embraces radical "change" ... I am sure the first engineers facing use of a PLC to build next year's automative assembly line reacted much as my great-great-grandfather reacted when he saw his first "indoor outhouse" (ie: first toilet inside of a home).

Modicon and Modbus are part of "history" in ways no other PLC or protocol are.

- LynnL