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Forum index » Science and Technology » Engineering » Control
Legacy PLC software and USB (slightly OT)
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bruce varley
science forum beginner


Joined: 31 May 2005
Posts: 38

PostPosted: Sat Jul 08, 2006 10:50 am    Post subject: Legacy PLC software and USB (slightly OT) Reply with quote

Really a software topic, but relevant to control:

I hear of problems with old instrumentation software using PC COM ports when
it's used with a newer PC which emulates the port using a USB adaptor.
Without any direct experience, I guess that the problem may be that the
newer arrangement uses a driver that make the port acceessible through
Windoze API calls (ie. looks like a file) rather than as a proper UART with
standard I/O port mapping. No doubt a lot of the older software would expect
a 'real' UART, accessing it via direct port read/write.

Is anyone aware of a robust solution to this problem, ie. a USB-based
solution that presents on the OS side as a dinky-di COM port, that can be
accessed like a real one? I've done a fair amount of googling and checking
of product specs, nothing is particularly specific on the matter. Suppliers
haven't been any help. TIA
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Cameron Dorrough
science forum Guru Wannabe


Joined: 09 May 2005
Posts: 103

PostPosted: Sun Jul 09, 2006 11:46 pm    Post subject: Re: Legacy PLC software and USB (slightly OT) Reply with quote

"bruce varley" <bxvarley@weqstnet.com.au> wrote in message
news:44af8de8@quokka.wn.com.au...
Quote:
Really a software topic, but relevant to control:

I hear of problems with old instrumentation software using PC COM ports
when
it's used with a newer PC which emulates the port using a USB adaptor.
Without any direct experience, I guess that the problem may be that the
newer arrangement uses a driver that make the port acceessible through
Windoze API calls (ie. looks like a file) rather than as a proper UART
with
standard I/O port mapping. No doubt a lot of the older software would
expect
a 'real' UART, accessing it via direct port read/write.

Is anyone aware of a robust solution to this problem, ie. a USB-based
solution that presents on the OS side as a dinky-di COM port, that can be
accessed like a real one? I've done a fair amount of googling and checking
of product specs, nothing is particularly specific on the matter.
Suppliers
haven't been any help. TIA

Situations like this generally require new software (a minor re-write with
the COM port calls changed to suit the OS) and can then run in Windoze and
similar 'multi-tasking' environments with few problems. USB allows the COM
port to be located elsewhere, but still doesn't present you with 'real' port
access because you're still restricted to the Windows API.

Fortunately it's really only data analyser software that needs to reliably
track individual data bytes that requires dedicated hardware. As I see it,
you have two options:

1. Replace the entire system (hardware and software) with something new.
Hey, if it's *that* old they are probably due for an upgrade anyway.. ;-)

2. Install dedicated hardware to suit the software. There are still
manufacturers of single-board computers out there (like Advantech) that can
supply DOS-compatible hardware with a standard COM port, an Ethernet port
and a 3.5" floppy. We've used this set-up to provide a gateway between the
old gear (Enraf tank gauges) and the new (DCS) calling it a "communications
interface". Didn't even need a hard-drive..

Your subject mentioned PLC software: We keep on old Toshiba laptop
installed with Windows 98SE in one corner of the office is loaded with the
old Mitsubishi, Modicon and Allen-Bradley software we need to support our
legacy clients. It doesn't get used very often and the battery was stuffed
long ago, but it doesn't take up much space either.

HTH,
Cameron:-)
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