Now I know Imust have a TTL to serial converter but there is no DB15 port out there, all of the products are DB9. And I don't want to use the DB9 to DB15 converter. How can I connect a serial DB15 port on arduino? and use it as a Modbus Master?
First of all , MODBUS is RS485 so you need a MAXIM MAX485 chip to interface with the arduino.
Second , there is no RS232 involved here so I don't know where that come from .
Third, the arduino is SERIAL TTL, "TTL-SERIAL " makes no sense.
You can talk about TTL-RS232 , TTL-RS485, etc but TTL-SERIAL makes no sense.
Fourth, the answer to your question about the connectors is a no-brainer. You buy a DB15 mating connector and solder wires to it
and plug the connector into your DB15 and plug the wires at the other end into the RS-485 to TTL chip (MAX485).