What is Host Link? How a PC talks to a KEYENCE KV series PLC

Host Link (上位リンク) is the communication function that lets a PC, touch panel or other external device access a KEYENCE KV series PLC. You connect over Ethernet and send a command written as text in a fixed format; the PLC returns or rewrites the value of a device.

The PLC (an EtherNet/IP unit or the CPU unit’s built-in port) is the server and the PC is the client. You do not need to write any ladder program for communication on the PLC side.

Commands are ASCII strings

Every Host Link command and response is an ASCII string. To read the value of DM100, for example, you send the string RD DM100 followed by CR (0x0D).

The PLC returns the value as text and ends it with CR LF.

A command may end with either CR or CR LF. LF is ignored, and anything after it is treated as the next command. Responses always end with CR LF.

Communication settings

The main specifications in the manual are:

8501 is the default and can be changed within 1–65535. Other ports have their own roles: 8500 is used by KV STUDIO, 8502 by VT (touch panels) and 5000 by the MC protocol.

TCP and UDP

With TCP, the PLC starts listening when it is powered on. Once connected, every command you send gets a response. When the client closes the connection, the PLC closes its side automatically and starts listening again.

UDP does not establish a connection. You send a command to the PLC’s IP address and port, and the response comes back. Only one peer can communicate over UDP at a time.

Available commands

Host Link commands are two or three letters.

The ones you will use most are RD, RDS, WR and WRS, which read and write devices: reading the production count in DM to show it on a screen, or rewriting an MR relay to change the operation, for example.

Which PLCs it connects to

The manual covers the built-in Ethernet ports of the KV-8000(A), KV-7500 and KV-5500, the KV-EP21V EtherNet/IP unit (mounted on a KV-8000, 7500, 7300, 5500, 5000 or 3000) and the KV-NC1EP for the KV Nano series. The device ranges and the number of points you can read or write at once differ by model.

Relation to the MC protocol

The KV series also supports the MC protocol (port 5000), which shares the TCP connections with Host Link. With Host Link you write KV device names (DM, MR and so on) as text, so you specify devices by the same names you use in your ladder program.

Using a library

KcpX for .NET builds these commands and parses the responses for you. You only specify a type and an address.

using KcpXLib;
using KcpXLib.Enums;

using var kcpx = new KcpX("192.168.0.10", 8501, series: PlcSeries.KV8000);
short value = kcpx.Read<short>(Prefix.DM, "100");

The next article looks at the command and response format in detail.

• Host Link command and response format