Editing Modbus (section)

==Other Modbus protocol versions==
Besides the widely used Modbus RTU, Modbus ASCII and Modbus TCP, there are many variants of Modbus protocols:
* ''Modbus Plus'' (''Modbus+'', ''MB+'', or ''MBP'') – Modbus Plus is proprietary to [[Schneider Electric]], though it is unpublished rather than patented, and unlike the other variants, it supports [[peer-to-peer]] communications between multiple clients.<ref>{{cite web |url=https://www.se.com/ca/en/faqs/FA198221/ |title=What is the difference between Modbus and Modbus Plus? |date=21 August 2004 |publisher=Schneider Electric |access-date=2017-02-07}}</ref>  Despite the name, Modbus Plus<ref>{{cite web |title=Modbus Plus - Modbus Plus Network - Products overview - Schneider Electric United States |url=https://www.se.com/us/en/product-range/576-modbus-plus/ |access-date=2014-01-03 |publisher=Schneider-electric.com}}</ref> is not a variant of Modbus. It is a different [[Communications protocol|protocol]], involving [[token passing]]. It requires a dedicated co-processor to handle fast [[High-Level Data Link Control|HDLC]]-like token rotation. It uses twisted pair at 1&nbsp;Mbit/s and includes transformer isolation at each node, which makes it transition/edge-triggered instead of voltage/level-triggered. Special hardware is required to connect Modbus Plus to a computer, typically a card made for the [[Industry Standard Architecture|ISA]], [[Peripheral Component Interconnect|PCI]], or [[PC Card|PCMCIA]] bus. Modbus Plus is normally implemented using a custom [[chipset]] available only to partners of Schneider.<!-- It is NOT compatible with simple [[RS-485]], despite some statements to the contrary. (Hidden because it seems to disagree with the reference)-->
* ''Pemex Modbus'' – an extension of standard Modbus with support for historical and flow data. It was designed for the [[Pemex]] oil and gas company for use in process control and never gained widespread adoption.
* ''Enron Modbus'' – another extension of standard Modbus developed by [[Enron]] with support for 32-bit integer and floating-point variables, and historical and flow data. Data types are mapped using standard addresses.<ref>{{cite web |url=https://www.simplymodbus.ca/Enron.htm |title=Simply Modbus - About Enron Modbus |publisher=Simply Modbus |access-date=2017-02-07}}</ref> The historical data serves to meet an [[American Petroleum Institute]] (API) industry standard for how data should be stored.{{Citation needed |date=January 2017}}

Data models and function calls are identical for the first four variants listed above; only the encapsulation is different. However the variants are not interoperable, nor are the frame formats.

===JBUS mapping===
Another ''de facto'' protocol closely related to Modbus appeared later, and was defined by [[Programmable logic controller|PLC]] maker April Automates, the result of a collaborative effort between French companies [[Renault]] Automation and [[Merlin Gerin]] et Cie in 1985: JBUS. Differences between Modbus and JBUS at that time (number of entities, server stations) are now irrelevant as this protocol almost disappeared with the April PLC series, which AEG Schneider Automation bought in 1994 and then made obsolete. However, the name JBUS has survived to some extent.

JBUS supports function codes 1, 2, 3, 4, 5, 6, 15, and 16 and thus all the entities described above, although numbering is different:
* Number and address coincide: entity #''x'' has address ''x'' in the data frame.
* Consequently, entity number does not include the entity type. For example, holding register #40010 in Modbus will be holding register #9, at address 9 in JBUS.
* Number 0 (and thus address 0) is not supported. The server should not implement any real data at this number and address, and it can return a null value or throw an error when requested.