Standard connection to a Pick-to-Light system

How is the standard connection of a Pick-to-Light system realized?

The connection of a Pick-to-Light system is realized via two separate channels with TCP/IP sockets. The connection is established via software that is installed on a server or PC. One channel is set up in each communication direction. The first connection is responsible for switching on the displays. The second interface reports back the acknowledgements of the withdrawals, the quantity corrections and the special keys.

The two channels work independently (asynchronously). The higher-level warehouse management system can activate or deactivate the displays at any time or change the displayed content or colors. Feedback is only provided on the second channel.

Securing data transmission

To secure the data transmission, an acknowledgement of receipt is sent by the receiver on each data channel (short acknowledgement). If the sender receives the receiver's acknowledgement of receipt correctly, the telegram is considered to have been transmitted.

If the transmitter does not receive confirmation of receipt, the data channel is closed, reestablished and the telegram repeated.

The telegram counter

Each data telegram is identified by the transmitter in the telegram header with a four-digit telegram counter (0001-9999). The telegram counter is used to synchronize the send and receive confirmation of the data telegrams. The receiver always confirms the receipt of a data telegram with the same telegram counter.

After the system start, the transmitter generates a telegram counter with the number 0001. The telegram counter is incremented with each data telegram and increased by +1.

After an overflow above 9999, the telegram counter is reset to 0001 by the transmitter.

The live telegrams

To check the status of the connection, the sender sends the receiver a live telegram (special telegram) without content. The telegram counter is always 0000.

The live telegram is only sent if no active telegrams have been sent in the channel within a certain time (usually approx. 30 seconds). The live telegram is acknowledged normally by the receiver (acknowledgement of receipt).

If the sender does not receive a confirmation of receipt from the receiver, the connection is closed and reopened by the sender. If the receiver does not receive any live telegrams from the sender, it also reinitializes its connection channel.

Who establishes the TCP/IP connection?

As a rule, the sender always establishes the connection. From this perspective, the first data channel (activation and updating of the displays) is established by the warehouse management system as a TCP/IP client. In this case, the Pick-to-Light system is a TCP/IP server.

For feedback, the Pick-to-Light system actively establishes the connection to the warehouse management system as a TCP/IP client. In this case, the warehouse management system is a TCP/IP server.

The two connections are kept permanently open and are only closed in the event of interface faults or system shutdown.

In some situations, the connection of a warehouse management system is only possible as a TCP/TP client. In this case, the Pick-to-Light system works on both channels as a TCP/IP socket server. The type of connection can be configured in our Pick-to-Light system.

What does the standard interface control?

A warehouse management system (WMS) can switch on, update and switch off the Pick-to-Light displays via the standard connection. The order picker's acknowledgements are reported back to the higher-level system.

With the standard interface, the entire logic of the order control, the mode of operation and the process is controlled by the warehouse management system.

The response times of the warehouse management system are crucial here. The response times should not exceed 500 milliseconds.

If the process is controlled directly by the Pick-to-Light system, the data transfer is implemented differently. Read more about this in a separate article on the customer-specific connection of the Pick-to-Light system.