Meldas 520 AMR, RS 232 Signal ground wire gets hot

[angelw]

Certainly could be a grounding issue. Specifically, it sounds like something on the CNC is not properly grounded and is connecting to ground through the PC.

If you have a multimeter, leave the RS-232 cable connected at only one end (either PC or CNC) and leave the other end disconnected. At the disconnected end, check for any voltage between the metal housing of the serial connector and the PC/CNC. Ideally, there should be no voltage. Check for both DC and AC voltage. I have seen machines in the field with as much as 30VAC between the machine and the PC.

I can't see that being a viable test. Unless the Shield Trace has been connected to the metal housing of the unplugged D connector, when its unplugged from the device, the metal housing is insulated from everything. I'd be inclined to do the test between the SG pin of the connector and the PC/CNC.

Regards,

Bill

[Caprirs]

That is a good point. I assumed the shield would be wired correctly.

[CNC Viking]

The shield is not wired to any D-sub pin. It is only grounded to chassis frame very near the CNC control in the cabinet.

[angelw]

That is a good point. I assumed the shield would be wired correctly.

Hi Caprirs,

There is much discussion regarding grounding of the shield, but I note that the Pin Out schematic early on in this thread shows no reference to the shield being grounded. Accordingly, I assumed that the OP's cable may not have had the Shield Trace connected to ground, as is often the case. Its also often the case that the Shield Trace is connected to pin 1, Protective Ground, of a DB25 connector. Protective Ground on a DB9 connector is the metal housing.

There are those that argue that the shield should be grounded at both ends (both devices), but to do so can lead to Ground Loops. As you no doubt know, a Ground Loop can occur when any two devices are grounded in Different Places, or if one device is not grounded well enough. For example, you may experience a ground loop when you connect a PC and a CNC control together with a serial data cable, if the CNC machine has a ground stake driven through the shop floor, and your Personal Computer is grounded via the ac wall socket, or if one device is not grounded well enough. The ground loop occurs when there is a different electrical potential between the Grounds of the two devices. This results in an electrical current being conducted through the data cable. and can burn out the IC chips that send and receive the RS232 serial signals, if the difference in potential is greater than about 30 volts. Accordingly, unless good grounding to the same ground can be assured for both devices, its recommended to ground the shield at the CNC end only.

I've come in quite late on the Thread, but after the quick catch up read I've done, I concur with you that its a grounding issue. This is somewhat reinforced by the fact that the issue is there when the Lap Top is powered via a power supply plugged into a wall socket.

Regards,

Bill

[CNC Viking]

Success!

Today I set up communication between the PC and CNC again, now with the Exsys RS232 Optical Isolator in between.

It really cuts all galvanic connections and worked like a charm even at 19200Baud as I always use.

The Isolator's 220V wall adapter use the same source as the PC.



The cable between the PC and RS232 Optical Isolator.



The cable between the RS232 Optical Isolator and CNC.

[A1CNC]

Glad to hear you have it working!
Have you tried drip-feeding @ 19.2 or just loading programs?

[CNC Viking]

On my other Feeler I do all machining by dripfeeding from a PC.
I always use 19.2 KBaud Baud and it works really well.

Will try dripfeeding the new Feeler soon, maybe tomorrow evening.

I only store a tool offset measuring/editing program in the CNC.
All CAM programs are stored in the PC + on a NAS disc.