According to a reference from 1996, **the vast majority of PLC communications is done via RS232C and twisted pair cables**.
Programmable Logic Controllers (PLCs) are the workhorses of industrial automation, controlling machines and processes. For them to coordinate actions, share data, or report status, they need to communicate not only with operator interfaces (like HMIs) and computers but also with each other.
## Historical Communication Methods
While modern PLCs utilize a wide range of network protocols, the provided reference highlights a foundational method prevalent in the past:
* **Primary Method:** RS232C Communication
* **Physical Medium:** Twisted Pair Cables
* **Maximum Speed (as per reference):** 19.2 Kbps
As stated in the reference: "The vast majority of PLC communications is done **via RS232C and twisted pair cables**. Most PLCs have an RS232 port and are capable of handling communications with host computers, printers, terminals, and other devices."
## Understanding RS232C and Twisted Pair
Let's break down this method:
1. **RS232C:** This is a serial communication standard. In simple terms, it sends data bits one after another over a single line (or pair of lines for transmitting and receiving).
* **Advantages (in its time):** Simple to implement, relatively low cost.
* **Disadvantages:** Limited speed (as noted, 19.2 Kbps), limited distance (typically up to 50 feet or 15 meters), primarily designed for point-to-point connections.
* **Application:** Often used for direct connections between two devices, like a PLC and a programming terminal, a PLC and a printer, or one PLC to another for simple data exchange.
2. **Twisted Pair Cables:** These cables consist of pairs of wires twisted together.
* **Purpose:** The twisting helps cancel out electromagnetic interference (noise) from external sources, improving data integrity over the cable.
* **Common Types:** Found in various network cables, including older RS232/RS485 setups and modern Ethernet. For RS232C, simple 2-wire or 3-wire connections using twisted pair could be used for data transmission.
## Practical Implications (Based on the Reference)
This communication method, while common in the era the reference describes, had certain characteristics:
* **Speed Limitations:** The stated maximum speed of 19.2 Kbps is quite slow by today's standards but sufficient for many control tasks that didn't require large data transfers or very frequent updates between devices.
| Feature | Detail (Based on Reference) |
| :--------------- | :-------------------------- |
| Communication Type | RS232C |
| Cable Type | Twisted Pair Cables |
| Maximum Speed | 19.2 Kbps |
* **Device Connectivity:** The RS232 port mentioned on most PLCs allowed them to interface not only with other PLCs but also with other crucial industrial devices like:
* Host Computers (for SCADA or data logging)
* Printers (for logging alarms or production data)
* Terminals (early HMI or programming panels)
## Summary
Based on the provided information from 1996, PLCs primarily communicated with each other and other devices using the RS232C serial standard transmitted over twisted pair cables, operating at speeds up to 19.2 Kbps. This method was prevalent due to its simplicity and effectiveness for the communication needs of automation systems at that time.
