possible to configure RS232 of UPS to Modbus using RS232 to RS485 converter


Thread Starter


I happened to see the support forum while checking the modbus on internet. I need a support for the purpose of converting RS232 of an UPS system to Modbus for BMS integration. I just purchased Aten RS232 to RS485 converter and also have ICP CON 7520 for connectivity checking in modscan software, but I am getting device time out error.

UPS RS232 9pinDBFemale connector-Adaptor (DB9pinmale to DB25male)-AtenIC485s converter-TX+ & TX- to Data+ and Data- of ICP CON7520-RS232 to PC. Please guide me where I went wrong and why I am getting time out error and also is it possible to integrate RS232 to Modbus by just using a RS232 to RS485 converter.

Please help me.
Modbus is a data protocol, not an electrical format. An RS-232 to RS-485 converter just converts the electrical signal. It doesn't convert the data going through it. The Modbus protocol itself doesn't care about whether you are using RS-232 or RS-485 (or Ethernet).

The problem here is your UPS. If the UPS itself doesn't understand Modbus, then it can't receive or send Modbus messages. You would need some sort of programmable "box" in the middle which will convert the messages which the UPS understands into Modbus. It is not possible to make any suggestions in that regards without knowing something about your UPS. Some UPSes do understand Modbus, but we can't tell if that is the case here with the information that you have provided.

Another potential problem is that a lot of serial UPSes (especially the simpler ones) don't actually send any serial communications through their serial ports when they signal power conditions (mains power off, shut down, etc.). They just use the serial port as digital I/O. You might be able to do something with this if your BMS(?) can work with digital I/O signals and you can put some suitable electronic voltage level converters in between the two.
An RS-232/485 converter cannot convert RS-232 to Modbus. Only a Fairy Godmother's magic wand can do that.

Modbus is a protocol, a set of rules for communciation that define who talks when and how the data is packaged and how to respond to errors.

RS-232 is a physical layer transmission standard for copper wire. RS-232 voltages have no clue as to what data in what protocol runs over or through it.

RS-485 is another physical layer transmission standard for copper wire.

An RS-232/485 converter converts from standard to the other, but does not change or affect the protocol running on the layer.

1) What leads you to believe that your unnamed UPS device is a Modbus slave which might respond to commands from Modscan, which, if Modscan is the Windows Modbus master program, could query the UPS device?

2) Since a PC COM port is natively RS-232; and your UPS seems to have an RS-232 port, you can probably get the PC to talk directly to the UPS over RS-232, in fact, isn't there a cable included to do that?



Zacharia, Tomy

Dear Ajay,
It is not possible by simply using an RS 232-485 converter.
Something more intelligent is required. An RS 232-485 converter only changes electrical characteristics of the interface.

Most UPS that reply on RS-232 do not reply using a protocol. Various pins are raised high to signal conditions such as battery low, battery critical, mains failed, battery faulty etc. In this case you would need a modbus device that can read individual serial port lines and interpret them to various coil status for reading in modbus. This might be relatively easy to implement.

If you have a UPS that speaks a protocol, then you are in trouble. You would need a protocol gateway that can speak UPS(ese) to the UPS and Modbus to your BMS system. So the first step would be to consult the UPS manufacturer regarding availability of protocol details and then be prepared to have a long talk with your modbus gateway vendor.

Tomy Zacharia
Sorry, but you're not the first person to be confused by this. RS232 and MODBUS are not the same thing.

- RS232 / RS485 is the HARDWARE that transmits messages. It says nothing about the message format
- Modbus is the FORMAT of the messages transmitted over a link. It says nothing about the hardware used.

Just because your device has an RS232 port does NOT mean it uses the Modbus message protocol.

Read the manual. If it says your device talks using Modbus messages via RS232 hardware then the answer to your question is Yes, otherwise the answer is No.

Yes indeed, the ups came with an RS232 interface cable and talks to the PC fine.
Here I would like to know with the existing RS232 UPS interface is it possible to communicate over modbus or read data in modscan. If possible how?.
Thank you so much for the valid reply
You said: "the ups came with an RS232 interface cable and talks to the PC fine". It would have helped a lot if you had simply told us this in the first place. You're not going to get useful replies if you hold back essential information.

If the PC will talk to the UPS over RS-232, but not over RS-485, then the problem is your RS-485 converter. The model you mentioned (ATEN 485s) supposedly has two switches to configure the converter. These must be set to the correct position in order for any communications to take place. ATEN does not provide any technical information about this product on their web site, only general sales literature. However, they say they do provide a manual with the converter.

What settings does the cable have, and what settings have you tried? What RS-232 configuration does GE use for the UPS? You've got to match the converter settings to both the UPS and to your PC (or BMS). I think you can set the pin configuration (DTC versus DCE) and also how the IC485 decides when its allowed to talk on the RS-485.

>>Here I would like to know with the existing RS232 UPS interface is it possible to communicate over modbus or read data in modscan.<<

Probably not. But you haven't provided enough information to answer your question.

You've established that your UPS talks some digital protocol (language) over RS-232 with your PC. That's all we know. We don't know what protocol/language it talks.

As others have pointed out as well, RS-232 is only the physical layer carrying the data. RS-232 does not define or imply what data travels over its wires.

We still don't know whether your UPS 'talks' Modbus or not. Some UPS models do, many do not. Forum readers have no crystal ball to divine what language/protocol your UPS talks unless you research it and come back and tell us. What does the documentation for your UPS say? What flavor of Modbus does it mention? Modbus RTU? Modbus ASCII?

If the documentation reveals that your UPS does not talk Modbus, then to get your UPS to talk Modbus you would need
- documentation of exactly what protocol and language your UPS does talk (not always easy to come by)
- a protocol converter
- the resources to program the protocol converter.

When you check out what that involves, the guy who signs the checks will typically ask, what can you do about using the PC that already talks to the UPS? Or what about about replacing the UPS with one that talks Modbus? Other forum readers might have other suggestions as well.

>Thank you so much for the valid reply<
The others were all valid replies, as well.

>UPS is GE make LP33 30KVA

GE LP33 30kVA manual on internet says it uses SNMP protocol as standard