When you looking to the simple and cheapest queue system, then this solution is what you looking for.
Technical specifications
1. At calling, the ticket number is flashing.
2. Lamp flashing 1s on 2s off during the employee or the clerk serving the client.
3. Lamp off during the employee or the clerk in a busy mode.
4. Lamp continues when the employee or the clerk in ready mode.
5. The data connection is RS-485.