Modbus RTU on Toshiba VF-AS1 VFD

[irmalcol]

I'm working on a project where we have to replace some Toshiba G3 VFDs. Currently, the G3 drives communicate with the PLC/DCS using Modbus RTU over a standard 2-wire interface (screw terminal block and shielded 2-wire cable). The cables are already run, and the client doesn't want to pull new cables, so I'm stuck using Modbus RTU. The recommended replacement Toshiba drive for the G3 is the VF-AS1. This drives will speak Modbus RTU, but it only comes with RJ45 ports. My drive vendor recommended using the XLTR-200 gateway from Industrial Control Communications to "convert" between the normal 2-wire interface to the RJ45 interface (but still using Modbus RTU). The manual for the XLTR-200 isn't very clear, and I am unsure whether it will actually work.

Does anyone have experience with using Modbus RTU with the AS1 drives or with the XLTR-200? Will this configuration work? Will it be a pain to use the gateway?

 

[danw2]

In the industrial world, one can never assume that the style of the electrical connector defines the electrical transport layer. RJ-45 connectors are used for RS-485 as well as Ethernet. Sometimes RJ-12's (former US phone line plugs/jacks) look like RJ-45's until you get up real close and look.

If the device 'speaks Modbus RTU', then I suspect that the connector that looks like an RJ-45 is not an Ethernet connector, but an RS-485 port for multi-drop Modbus RTU functions, but you need to go to the documentation to confirm one way or the other. There might be a comms manual that covers it in more detail than the users manual. The spec sheet might also confirm that a given model number has a specific comm port.

Although some devices are technically talk Modbus-RTU-over-Ethernet rather than Modbus/TCP, the distinction isn't made clear, so the normal assumption is that Modbus RTU refers to the serial protocol that runs point-to-point on RS-232 (at distances up to 15-25m) or multidrop up on RS-485 to distances up to 1000m.

 

[irmalcol]

Thanks, danw2. I completely agree with you, and you are spot on. Perhaps I wasn't clear. The AS1 has an RJ45 jack via with which the drive can speak several different protocols with different signal standards. Modbus RTU over RS485 is one possibility and the one I am interested in. There are two issues:

1. The AS1 drive uses so-called 4-wire RS485. It is setup to have 2 wires for Tx and 2 wires for Rx (enables full-duplex communication). The existing plant wiring is what I'll call standard 2-wire RS485 (half-duplex). I need to "convert," if you will, between the two types.

2. I need a way to physically convert between the 2-wire screw terminals to RJ45. Both accommodate Modbus RTU over RS485, but there is the physical rewiring that has to be done. I don't want to make my own cables with RJ45 on one end and loose wires for the screw terminals on the other.

I have been told that the XLTR-200 can solve both of these problems for me, but I'm skeptical. Has anyone done this before?

 

[danw2]

>The existing plant wiring is what I'll call standard 2-wire RS485 (half-duplex). I need to "convert," if you will, between the two types.

2 wire RS-485 is half duplex by definition. It cannot be converted to full duplex. The 4 wire 485 can be jumpered [(+) to (+), (-) to (-)] to operate in half duplex mode, but there could be an issue about the protocol expecting to run full duplex and not knowing the wiring's been jumpered.

If you Google 'RJ45 screw terminal' you'll find lots of adapters

 

[irmalcol]

Yeah, I realize that. That's why I need this gateway, since it's designed to work with both half-duplex RS485 and specifically with Toshiba drives.

 

[djs]

Sometimes the drive can be software configured to be half duplex. This is even though there are separate pairs of receive and transmit terminals. When configured as half duplex, you tie the receive and transmit pairs together. If you can't configure the drive you will have to use some kind of converter or gateway.

 

[danw2]

I'm skeptical, too, because the gateway doesn't even appear to have an RJ-45 for RS-485.

Why don't you call the ICC people directly. I've generally found the gateway people very upfront about what works and what doesn't.

 

[FreddyNurk]

RS485 via RJ45 connectors, is not, as I understand it, any sort of industrial standard, though from a product point of view its far easier to provide an RJ45 connector rather than screw terminals.

Red Lion is one such manufacturer that does it, and I'd expect that the pinout for the RJ45 would not be anything like the devices you mentioned, thus the absence of an RJ45 port on the recommended device doesn't mean that it wouldn't work. Red Lion also sell breakout terminals for RJ45 to 8 way screw terminals, but they're not that cheap either.

From an ease of installation and elimination of wiring errors point of view its quite frustrating to have to bring both sets of wires out and connect them manually, but that's probably what it would take.

A couple of options, depending on your availability of test gear and similar: (these assume that you've got access to the gear in question such as the drives...)

Modscan32 (there's a new 64 bit Win7 version available, though I've not had the best of luck with it) can be used with a PC and an appropriate converter (FTDI make chips for a number of RS232 and RS485 converters that work with most PCs, they're available from Dontronics among others), will allow you to test the drive's response.

IFTools make some awesome serial line diagnosis units that can tell you what's actually being sent and received along the serial line, but they're not cheap. We used them to diagnose some failed serial cards in some aged equipment that we had, and would likely not have been able to diagnose it without them.

Of course, all this is easier said than done if you don't have access to the gear to test it. If thats the case, I'd be pushing it back onto the vendor. The biggest issue that you're likely to find is that the gear is full-duplex and it can't be supported on a half-duplex implementation.

I don't have any affiliation with any of the companies mentioned.