What standard was introduced in the early 1980s to enable communication between machines from different manufacturers?

The standard that was introduced in the early 1980s to enable communication between machines from different manufacturers is MODBUS. This protocol was developed by Modicon (now part of Schneider Electric) to create a standardized way for devices, such as programmable logic controllers (PLCs), to communicate over serial lines.

MODBUS is particularly significant because it allows for devices made by different manufacturers to work together seamlessly, providing a common language for communication. This interoperability has made it a popular choice in industrial automation and monitoring since its introduction.

While TCP/IP and Ethernet are protocols that enable networking and data exchange over broader networks, they did not specifically address the communication needs of industrial devices until later integrations. RS-232, on the other hand, is a standard for serial communication but does not provide the higher-level protocols needed for interoperability like MODBUS does. Thus, MODBUS stands out as the correct answer in this context.