Honeywell HC900 and ML50


Thread Starter



Factory A:
I have 5 compressors running with ABB inverters. The inverters have modbus RTU built-in. They are monitored and controlled by HC900 via modbus RTU communication.

Factory B:
I have 2 compressors (stand-by) operating with Danfoss softstarters (also have modbus RTU built-in). They are controlled by ML50 so far via modbus RTu communication.

The distance between factory A and factory B is about 100 meter (open air).

The question is: how can I connect both factories into one controller i.e. the HC900. I wish to switch on the starter(s) if the compressors in factory A are overloaded.

HC900 still has Ethernet and RS232 ports. ML50 only has RS232 port.

Thanks guys,

Patrick Lansdorf

Hi Keith,

I work for a company that makes gateways, HMS.

To me it sounds as if a gateway could be a way to go. I assume that both the HC900 and ML50 are Modbus RTU masters and that you use some kind of RS232 to RS485 converter in order to multidrop.
Solution 1. Use a ModbusRTU Slave to ModbusRTU slave gateway, to connect the two masters: X-gateway

Solution 2.If the ethernet port of the HC900 can communicate via Modbus TCP or EtherNet/IP, a Modbus TCP slave to Modbus RTU slave could be used. This requires that Factory A and B are connected on the same ethernet network. X-gateway

Of course there might be other solutions, such as OPC etc.

If you have any questions regarding the gateway solution, let me know at: support [at]

//Patrick L
Presumably, the ABB drives are Modbus RTU slaves. On that assumption, the HC-900 is a Modbus RTU master to the ABB drive(s) using either its RS-485 port (since the RS-232 port is still open).
I believe (could be mistaken) that the HC900 can be a Modbus Master on only one of its comm ports, whether that comm port is the Ethernet, RS-232 or RS-485 port. However, the HC's Ethernet port can do dual duty, acting as a combination Modbus slave or Modbus master; in a sequential fashion.

I don't know what an ML50 is. I'm guessing that the ML50 is most likely a Modbus master to the Danfoss softstarter (Modbus slave). If so, have the ML50 read whatever Danfoss data is needed to ML50 holding registers. Then

- the ML 50 (as a Modbus master) could write the Danfoss data to the HC900 (Modbus slave)

- The HC (as a Modbus master) could read the Danfoss data from the ML50 (as a Modbus slave)

Since you don't have Ethernet on the ML50, you could try the RS232 link between the ML50 and the HC. There shouldn't be any problem assigning the HC as a second slave to the ML50, the HC's serial port is fully configurable. At 100 meters, you're pressing beyond the 'specification' distance limits of RS-232, but last week I got a half duplex RS-232 link running at that distance with CAT5e cable. For the cost of the cable, it's worth a try, because both the HC and ML50 have an available RS-232 port. Of course, you can always get a pair or RS-232 to 485 converters, one for each end and do it 485, too.

Or, pursue the ML50 Ethernet upgrade and do it Modbus TCP.

Hi David;

Thank you for your valuable information. Do you have the model of the ML50 Ethernet upgrade. Seems that you are familiar with Honeywell products.


wayne preston


As already stated the hc 900 will act as a master and/or slave on all ports, also the ports S1 and S2 are configurable as rs485 ports or rs 232 ports.

perhaps you could wire the second PLC/device straight into the hc900 and poll the compressors status directly as required.


Sorry, I can't help on facts about ML-50. Whatever the ML-50 is, I've never seen or heard of it here in the US. No support documents for the ML-50 appear on Honeywell's public US web site.

It's a mystery box to me, other than it must be a Modbus master to talk to the Danfoss drives which are Modbus slaves.

Hi Keith,

HC900 does support Modbus/TCP so the suggestion from Patrick L to use a Modbus/TCP to Modbus RTU gateway is a good one. Be aware, however, that 100 meters is at the upper limit for Ethernet over copper.

HC900 can not be a Modbus master on both serial ports at the same time, but it can be a master on serial and Ethernet simultaneously.

I hope this helps.