A very basic question!

When connecting to a plc with a laptop using a serial communications cable, is there a "preferred" order of connecting the cables. Is it ok to have the laptop on and booted up before connecting the cable to the pc or should you have the pc off until the connections are made to the pc and the plc. Is there any concerns with connecting the cable to the plc with the plc in run mode and the machine in operation. What about connecting your laptop to a printer with the printer and the pc powered up. I was told you can damage the parallel port if both devices are on when connecting the cables. I know these are "basic" questions, but other "newbies" must be wandering as well with these issues. I was referring to Allen Bradley and Modicon plc's if it matters any!!!

The only thing I would suggest is that you dissconect your link from the PLC before pulling on the cable. Other then that I never muched paid attention to how I connect or disconnect the serial cable PC on/off or whatever..never had an issue with it.

As far as the parralell cable goes..when I had one, I usually turned off the equipment before making or breaking the cable connections.

BTW..I'm not an AB user..

The only trouble I ever got was pulling the cable while on line. Some PLC programs will franticly thrash around for a minute or so, using all your PC resources, looking for the disconnected PLC. The worst one for this was Concept. Other than that, no trouble plugging or unplugging the connection.

Doug

I usually avoid having the PC serial port connected to the PLC when I re-boot. On some plarforms the PC sends inquiries to the serial port looking for a mouse, and this causes some comm lockups and link malfunctions. I haven't noticed it with XP, but then again I still try to pull the plug when I reboot out of habit.

Also, not for plc's specifically, but some of the older servodrive controllers(kollmorgen bds5 specifically) will actually fault the drive sometimes if the cable is disconnected before the programming software is shut down.

i always say to my self,

dont disconnect any cable while it being use for communication.

REZAR,
Talking to yourself? This stuff will do that to you sometimes.

My personal procedure is to:
Connect comms lead to live PLC then PC prior to boot-up of PC. Some serial connectors have grounded D-sub assemblies so this is grounded firstly at the PLC.

Upon completion of work ensure no comms between PLC<>PC, such as RSLinx, no monitoring in progress, etc... Disconnect from PC then PLC.

Have read somewhere that power used by PC should be off the same phase as the power to the PLC... That has just set me the task for the day to find out where I got that info and why it is so.

With my laptop, I have to use one of those Serial-to-USB adapter cables, since it has no serial port. I've noticed that I need to have the adapter cable plugged into the USB port on the laptop before powering up the laptop, or it won't be recognized.

1) Plug adapter cable into USB port.

2) Power up (or re-boot) laptop.

3) Connect PLC

4) Power up PLC

5) Open software program

This is for A-D PLC, by the way, and IBM ThinkPad

Paula

An unusual exception

Okay, Wiley, I'm going to throw in a circumstance where the order of connection does matter.

If you are running a DH-485 network and using a 1761-NET-AIC module to connect your PC to the network, make sure that the RSLinx "1747-PIC/AIC+" driver is configured and running BEFORE you connect the PC to the 1761-NET-AIC.

On some computers, with some operating systems (I have heard it the most with Windows NT 4.0, but sometimes with Windows 2000 and XP too) the PC will force the RTS pin high when the port is unconfigured. If such a port is plugged into a DH-485 network with a 1761-NET-AIC, the PC will in effect take over transmission on the network and not relinquish it, causing the token passing to fail and the network to "crash".

This problem should fade as the 1747-UIC is adopted, but it can bite you if all the factors coincide.

I was under the impression from siemens literature that coms cables should always be connected before powering up devices , this is particularly relevent when using a non grounded chassis pc like a laptop - I am trying to dig out the reference to it .
The PLC is relatively safe , but there is the danger of cooking the com port - siemens waffle on about an auxilary ground lead in some cases.

This isn't exactly what was asked, but some years ago I came across an example of cable usage which caused many problems.

This was a site owned by an American company (hint : number between 2 and 4; and letter between L and N). They had a PLC installed in a locked cabinet, and to prevent the need to open the cabinet for programming or monitoring, they had attached a small RS232 lead from the programming port to a 25-pin D-type mounted through the cabinet door. Effectively they had just extended the programming port's location by a few inches. So far, so good.

However the staff had then screwed a 4-foot long null-modem cable to this 25-pin port on the outside of the cabinet. That meant whenever they needed to attach a programming device, the cable was always there. And whenever no-one was using the PLC, the cable was just left, unterminated at one end, dangling down the front of the outside of the cabinet. Nearby on this production floor were a couple of machines with ultrasonic welding heads for plastic bags. These things are, I understand, notoriously good generators of RF noise.

After several occasions when the PLC went in to a fatal error, someone eventually had the good idea that it might be related to this cable. It was acting as an antenna for all the RF noise in the air. Take the cable away, problem disappeared. Now, agreed, it was a badly made cable, poorly-shielded, but the memory has always stuck with me. So, ever since then whether it's rational or not, I always tend to connect to the PLC last and disconnect from it first. I try not to leave unterminated cables attached to my PLCs - you never can tell.

Ken