RS485-4 and RS485-2 what to do?

Hi,


I am working with a Mitsubishi VFD that has RS485-4 communications. Another servo in the system has RS485-2 and Ethernet.



My communication need for the Mits VFD is only to write parameters to the VFD like speed or torque reference and maybe jog speed reference.


I know this subject must have been beaten to death over the years. But what do you do in this situation us a converter?
Is there a wiring way to write only to RS485-4 wire from RS485-2 wire?



On my HMI I have many options. The HMI has RS485-4 or RS485-2 (but not both at same time), a separate RS232 port along with Ethernet and USB host.



What is the easiest way with least amount of additional hardware to communicate with the RS485-4 port? I guess I could look at getting an option card to give the Mits FR-V500 ethernet and then I could just use all ethernet.


TIA

http://www.plctalk.net/qanda/showthread.php?t=77499

The above thread may help you out.
Regards,

I have found that one and another on the Google (RS232 to RS485-4). But what I am think maybe the easiest was is use the HMI RS485-4 and then use the RS232 with a RS232 to RS485-2 converter like BnB has, just not sure if it can do multi drop on the RS485-2 side... and their site is down right now.

2 wire RS-485 can definitely be multidrop (assuming the protocol supports multidrop).

RS-232 is intended for point-to-point, with no multidrop functionality, but if the protocol being implemented carries the needed data for multidrop, then conversion to 232 might work, but I'm not sure why you'd want to bother.

Because if I use the RS485-4 port on the HMI I can't use the RS485-2 port, but I have and RS232 port available on the HMI. So if I convert I can still control RS485-4 and a RS485-2.


The only thing is the converters out there ($15-20 US) are designed for a PC RS232 to RS485-2, not sure if it will work from a HMI RS232 port? I think for the $20 I will get one just to play around with anyway.

danw said:

2 wire RS-485 can definitely be multidrop (assuming the protocol supports multidrop).

RS-232 is intended for point-to-point, with no multidrop functionality, but if the protocol being implemented carries the needed data for multidrop, then conversion to 232 might work, but I'm not sure why you'd want to bother.

Click to expand...

RS422 uses two pairs of wires, one for transmitting and the other for receiving, and this is what I think you are calling RS485-4, it is RS422.

RS485 uses a single shared pair, the same pair is used for transmitting and receiving.

There are devices that are able to communicate with both RS422 and RS485 usually they have 4 connection terminals TX+, TX-, RX+, RX-

In the case of using RS485 then TX+ and RX+ are connected together and the same with TX- and RX-

lfe said:

RS422 uses two pairs of wires, one for transmitting and the other for receiving, and this is what I think you are calling RS485-4, it is RS422.

RS485 uses a single shared pair, the same pair is used for transmitting and receiving.

There are devices that are able to communicate with both RS422 and RS485 usually they have 4 connection terminals TX+, TX-, RX+, RX-

In the case of using RS485 then TX+ and RX+ are connected together and the same with TX- and RX-

Click to expand...

Actually this is a common misunderstanding. RS422 and RS485-4 wire are different. RS422 is a 4 wire communications layer (5 wires but that's a different discussion) with two pairs of one-way communications I.E. each pair sends data in the opposite direction than the other. RS485-4 wire also has two pairs of communication wires but they both are bi-directional meaning that each pair can send or receive data if the equipment using it utilizes that functionality which most does not. It is worth noting that a vast majority of equipment out there that uses RS485 uses the two wire version which is half duplex only and when needing full duplex RS422 gets used. Because of this, somewhere along the way, the perception became that RS485 is two wire and RS422 is four wire.
Is this important to the conversation? Yes! Because it's labeled RS485-4 wire (assuming that it is) it's possible that the equipment is utilizing two paths of half duplex, bi-directional communications and if that is the case, you won't be able to convert it to RS232 or RS422 unless you have a converter that is specifically designed to convert RS485-4 wire with bi-directional communications to something else which is kinda a tall order. Unfortunately I think you might have some more digging to do info wise and find out what exactly is happening on the four wires. Having said all of that, it is very likely that while it is RS485-4 wire that it operates without using the dual bi-directional communications which will make converting it easier.

If you mean you have a 4wire rs485 device and a 2wire rs485 device then you can connect the 4wire as stated by Ife. You can then connect upto 32 devices on the network without a repeater. Each device will have its own modbus address.

cookie_cookie said:

If you mean you have a 4wire rs485 device and a 2wire rs485 device then you can connect the 4wire as stated by Ife. You can then connect upto 32 devices on the network without a repeater. Each device will have its own modbus address.

Click to expand...

I would check with Mitsubishi first before wiring it that way. If it needs full duplex communications that won't work.