I had SERIOUS problems with my X200 VFD and RS485 bus- I "mostly" fixed it.
Here's the thing- yes, RS485 is differential, but the VFD is probably NOT opto-isolated input. Differential conveys strong noise immunity- but ONLY when both A and B wires' voltages are within the input range of the VFD's bus driver, which might be -0.5v to 7v relative to the ground on the VFD driver chip. Outside that range, it will NOT function. The X200 confuses me greatly. Yes is has an A and B RS485 terminals, but no ground on the RJ45 jack. If you don't have a ground to connect to, this problem can easily come up, and DID. I had to take apart the VFD to measure this- like >20v of noise between the PC ground and the VFD driver. That will NOT work, and didn't. You might think "there's that 'L' terminal in front for 'Logic Ground', I'll tie the RS485 ground together there". But I measured, it's not tied to the RS485 driver's ground- which is confusing and weird but that's what it is. Clearly Hitachi did NOT intend for it to be ground, since it's not on the RJ45 jack. It'd be awkward to have a removable RJ45 and a nonpluggable ground. I ended up using an ADAM isolated RS232-RS485 bridge (needed to get RS232 to RS485 anyhow). This is still questionable because I STILL only floated the RS485 ground- but that's better than having it referenced to something guaranteed to be incompatible with the X200's ground. Snooping the bus driver, the Modbus does get a bad transaction like 10% of the time but that's good enough. I honestly don't know how Hitachi intended for that X200 to work. In the WJ200 manual, they show "L" IS supposed to be the ground for RS485 comm, though. I went through the manual and can't find the pinout for the RJ45 jack, I don't know if they included a ground or not. I guess that's what they intended for the X200 too, to use the "L" as gnd? But anyhow- The X200 and WJ200 at least have optional internal termination resistors. But you're only supposed to build a modbus in a line and only the end device can have a termination resistor. "Quality" of wire will NOT matter much especially in short lengths like <3m or whatever. Danny On 3/30/2016 11:13 AM, andy pugh wrote: > On 9 February 2016 at 11:23, andy pugh <[email protected]> wrote: >> This is where things get iffy. Imagine that I have the 2-terminal >> dongle plugged into a laptop. What do I use for a gnd? Stuff a wire >> down the side of the USB connector? trap the wire in the hinge? :-) > In the end, I fitted a different VFD which has a pre-written HAL driver. > It also only has two connections for RS485, so is less confusing. > I found a way to connect the dongle to the motherboard header: > https://forum.linuxcnc.org/forum/18-computer/30675-on-motherboard-modbus-rs485-connection > They are so cheap that I was prepared to go through a couple > experimenting, but it works fine. > ------------------------------------------------------------------------------ Transform Data into Opportunity. Accelerate data analysis in your applications with Intel Data Analytics Acceleration Library. Click to learn more. http://pubads.g.doubleclick.net/gampad/clk?id=278785471&iu=/4140 _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
