# Modbus Serial to Modbus TCP called from external master
**URL:** https://discourse.nodered.org/t/modbus-serial-to-modbus-tcp-called-from-external-master/19397
**Category:** General
**Created:** [20 December 2019 14:47 UTC](https://discourse.nodered.org/t/modbus-serial-to-modbus-tcp-called-from-external-master/19397 "2019-12-20T14:47:33Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@bryans](https://discourse.nodered.org/u/bryans)
#### Post date: [20 December 2019 14:47 UTC](https://discourse.nodered.org/t/modbus-serial-to-modbus-tcp-called-from-external-master/19397/1 "2019-12-20T14:47:33Z")
I have been working on this for quite some time now and cannot quite get it to work properly. Here is my scenario:
I am reading Carbon monoxide levels with a sensor that communicates modbus over RS485.
I am using a Raspberry Pi 4B, which is connected to a bb smartworx RS485 to USB Converter.
I am able to read the desired registers with a "modbusSerial" node.
That being said, this is where I get myself confused, I need to poll the Flow from my SCADA system from a remote server (This will be a modbus request)
the way I visualize it is here:
[SCADA sends modbus TCP request for data from Carbon Monoxide sensor]
therefore
[SCADA sends modbus TCP request to node-red, node-red requests serial modbus data from Carbon monoxide sensor, node-red takes response and passes it along to SCADA in response to the original call]
I am fairly new to node-red but I am catching on pretty quickly. I am confused as to how I should handle this, any help would be greatly appreciated.
---
### Author:  [@ronaldogm6](https://discourse.nodered.org/u/ronaldogm6)
#### Post date: [3 October 2020 21:39 UTC](https://discourse.nodered.org/t/modbus-serial-to-modbus-tcp-called-from-external-master/19397/2 "2020-10-03T21:39:19Z")
@bryans Hi , i wondering did you solve this problem . I have problem i want using pl2303 rs485 to usb converter, but i cant write on the USB using node red. Do you mind sharing how to solve this situation ? Thank you