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<I> MarK V Modbus RTU with DCS
Modbus communication losing queries randomly after <I> replacement


We have recently replaced one <I> on a MKV system. The new <I> is based on the HP Netserver E200 motherboard, the old was the original 486 supplied with the panel.

On the new <I> we are having issues with the Modbus communication. It appears that the <I> sometimes does not answer to the Master Probes. This appear to happen approximately 1 time every roughly 200 queries. Any idea of what may cause this?

The new <I> has had IDOS copied to it from a previous image disk of a different site, and then has been reconfigured with the local config directory.

Best Regards



Copying IDOS from another system shouldn't really cause a problem--BUT, IDOS was written for very specific versions of BIOS chipsets and it doesn't always work well with just any BIOS on a compatible motherboard. (meaning, a motherboard with ISA slots for an ARCnet card for the StageLink). Sometimes it's as simple as the keyboard not working; other times it's intermittent problems with functions like MODBUS. Frankly, not knowing anything about the vintage/version of HP Netserver motherboard/CPU it's very surprising that you got it to work at all. (Could you provide more details about the CPU/motherboard?)

I presume the MODBUS connection is via serial communications (RS422 or RS485). It could be a problem with the serial COM port set-up on the motherboard.

But it's most likely an IDOS BIOS incompatibility issue. GE did not have the time or the resources (manpower and money) to qualify every BIOS chipset available at the time. And, since IDOS is no longer supported they don't do any compatibility testing any longer that I'm aware of.

I believe the only BIOS chipsets IDOS was intended to work with were manufactured by Award and Phoenix--and very specific versions of chipsets within those two manufacturers. I know early Compaq chipsets were not compatible at all with IDOS, and some IBM computers were also not compatible (though a work-around was written for that since many field engineers had IBM notebook PCs at the time).

Check your motherboard's COM port configuration, in both the IDOS files (F:\OPTIONS.DAT) and on the motherboard. You might try switching to a different interrupt for the COM port, switching COM ports, a combination of the two or something like that. You may have to slow down the link, also. (IDOS did some wonky things with interrupts, which was usually why it wasn't compatible with many BIOS chipsets.)

In general, though, MODBUS communication issues with <I>s were usually caused by problems at the other end of or somewhere else along the link. If the link is a 'multi-drop' link it may be some issue somewhere else along the link, or it could just be a wiring/cabling issue.

Hope this helps. Please write back to let us know how you fare in resolving the issue and if you resolve the issue what you did to resolve the issue!

The mother board is an ASUS p3v133. This one works fine with the Arcnet and has shown no issue apart form this hiccup on the Modbus.

I've tried changing the speed but without any real success.

The requests themselves are quite calm about one query per second.

I'm confident that the issue is on the <I> as we have a test system with the same motherboard and when configured like this one also that shows the same issue.

will try with a different version of IDOS, and also changing the COM port.

Thanks, will keep you posted.


Thanks for the feedback!

>will try with a different version of IDOS, and also changing
>the COM port.

Why aren't you using 4.7--I believe that was the last version of IDOS GE released. I would expect the last version released would be the most recent version.

I believe if you look in F:\IO_PORTS.DAT you will find some information about configuring Master and Slave ports.

And, I believe in the BIOS setup for the motherboard you can find which interrupts are defined for the COM ports. Some BIOS chipsets use varying interrupts, different from other chipsets.

This is all I can think of. Perhaps someone else with some MODBUS configuration experience on <I>s could be of more help.

Hello again, about the version of IDOS.

On the log file on the new <I> I'm getting IDP0046, Version 4.6 of <I> PRODUCT CODE a bit below

In reference to the 4.7 you are talking there is it the first one [so I'm behind just of one minor release] or the second one [and I'm QUITE behind].

Additionally what would be the best way to get the latest release?