CN1 Terminal Connector in Two Wire Half Duplex Mode for RS-485

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General Background[edit]

This article is quite specific in its scope as it is about using CN1 Terminal Connector to communicate through RS-485 (Now known as EIA-485). Usually Horner APG’s NX OCS PLCs feature two active serial ports which are accessible through three connectors; Port 1, Port MJ1 and Port CN1. Therefore, its applicability is very much specific to limited circumstances but its importance is very much crucial as it serves a very important purpose.

Introduction to PLCs[edit]

A Programmable Logic Controller (PLC) is an industrial computer which is used to monitor inputs and depending upon its state, a PLC makes decisions based on its program or logic to control its output to automate a machine or a process. In an automated system, a PLC is the central part of the process control system. It is possible to connect more PLCs to a central computer in order to run more complex processes.

Communication with PLCs[edit]

PLCs communicate with various devices using different Protocols. The control of the data between two devices is very much necessary in order to define what constitutes the communication and how the communication is to be initiated and terminated. This is called the protocol. Here, one device needs to indicate to the other to start or stop sending the data.

In order to facilitate the communication between devices the International Organization for Standardization (ISO) devised a model to be used for standardization for Open System Interconnection (OSI) and accordingly, there are various protocols in place including Modbus (Modbus TCP/IP, Modbus RTU, PROFIBUS, ControlNet, DeviceNet, etc… And they make PLCs communicate using serial communication or parallel communication or Ethernet as the mediums of communication.

Modbus Communication[edit]

Serial Communication:

Generally, electronic data communication between the elements falls in two broad categories; Single ended and Differential.

The example of Single ended communication is RS232 (it is now known as EIA-232). It is one of the three physical layers for Modbus. And as understood from the word “single ended” it allows for the data transmission from one transmitter to one receiver, in other words from one master to one slave and that too at relatively slow data rates of up to 20K bits/second. It employs a cable with three conductors, Tx (Transmitter), Rx (Receiver) and Ground, with maximum length of a couple of hundred feet.

Differential data transmission offers superior performance when communicating at high data rates or over long distances in the real world environments. The example of differential data transmission is RS485. It allows the data transmission between one master and up to 128 slaves but care should be taken to read more if there are more than 32 slaves. Here, the data speeds are up to 15K baud (bits/second) and the maximum physical length that can be served is 4000 ft. The reason behind the word “differential data transmission” is that here in this system of communication, the transmission takes place due to differential of voltage difference between the lines (cables).

It is also useful to understand that there are two types of wiring systems; “so called” 2-wire systems and and 4-wire systems. These are incompatible but usually 4-wire systems can be made to work on 2-wire systems. So, RS485 is a 2-wire standard, but 4-wire systems can be made to work on it.

Ethernet (TCP/IP):

This communication system is employed by Modbus TCP and here, all the devices are peers. In this system, a single device can be a master and a server. Subnets can be connected together by routers.

Case Study: Horner APG's NX OCS PLC[edit]

In the case discussed here, one of the customer’s requirements is to integrate Horner APG’s OCS/NX PLC (Protocol: Modbus) on to BACnet IP using Fieldserver (gateway) to carry out the integration.

Now, Horner APG’s OCS/NX features two active serial ports, accessible through three connectors. There are three physical ports (Port 1, Port MJ1 and Port CN1) out of which two ports (Port 1 and Port MJ1) share internal circuitry and consequently, these two ports can not be used simultaneously. However, Port CN1 has independent circuitry and it can be used simultaneously with either Port 1 or Port MJ1.

And in present case, customer wants to connect using CN1 Terminal Connector through RS485. Now, CN1 Terminal Connector is shown below in figure – 1.


Figure - 1: CN1 Terminal Connector

[Technical Data Sheet on Horner OCS/NX, {Horner APG}]

Table – 1 below elaborates the pins on this CN1 Terminal Connector

Pin Signal Direction
1. TX+ OUT
2. TX- OUT
3. RX+ IN
4. RX- IN
6. GND -
9. +5V OUT
10. N/C -

Table – 1: Pins on CN1 Terminal Connector

(Table reproduced from the source mentioned below)

[Technical Data Sheet on Horner OCS/NX, {Horner APG}]

In table above,

TX – Transmitter

RX – Receiver

TXD – Transmitted Data

RXD – Received Data

GND – Ground

As it appears, there are two TX and two RX connections on CN1 Connector, which means it is 4-wire system. Here, the data can be received and transmitted simultaneously through this connector. This kind of arrangement is called 4 – wire full duplex mode for system communication, in which, two way data communication is possible simultaneously.

But the customer wants to connect CN1 connector through RS485, and RS485 is a 2-wire system.

Now, to connect through RS485 with CN1 terminal connector, it is possible to add the “Jumpers” across TX+ and RX+ and across TX- and RX-. Adding the jumper means to physically “twist pair” TX+ and RX+ and TX- and RX- and by doing so, make them behave as two single wires on the network.

What it does??

Well, by doing so, now, there are two wires for data transmission and data receiving, in the communication, instead of 4 wires initially, i.e. TX+, TX- and RX+, RX-. Therefore, now it is possible to connect CN1 terminal connector through RS485.

However, by connecting CN1 connector through RS485, and converting 4 wire system to 2 wire system, puts constraint on the data communication. Initially, two way simultaneous communications are possible through CN1 connector via TX+, TX- and RX+, RX-, i.e. 4-wire system. So, originally, the system is in 4-wire full duplex mode.

But, after adding jumpers across TX+, RX+ and TX-, RX-, there are two wires available for data communication in the system, and consequently, two way simultaneous data communication is not possible in the system. The data can be either transmitted or the data can be received at a time. This kind of an arrangement is known as 2-wire half duplex mode for a system.

And in the present case with this particular customer, CN1 terminal connector is utilized in 2-wire half duplex mode for connection through RS485. And it can be learnt from this, that whenever such condition arises, CN1 terminal connector can be used in 2-wire half duplex mode for connection through RS485.

See Also[edit]

1. Modbus

2. EIA-232 (RS-232)

3. EIA-485 (RS-485)


1. PLCs: Introduction
2. PLC Communications
3. ISO/OSI Protocols
4. Quick Reference for RS485, RS422, RS232 AND RS423
5. Frequently Asked Questions for RS485, RS422, RS232 AND RS423
6. Modbus for Field Technicians by Chipkin Automation Systems Inc.
7. Technical Data Sheet on Horner OCS/NX, [Horner APG]