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I am looking to get a whole bunch of digital output pins that I can directly control from my CPU and I am hoping to use a whole bunch of USB to serial ports to achieve this.

Assuming I am using a high end computer with a modern OS (Core i7 860, Windows 7 x64, plenty of on board USB ports). Is there any practical limit to the number of these devices that I can attach to the computer and still use properly? I ask this because computers usually never had any more than 4 COM ports on them, but this was back when computers actually had COM ports on their motherboards.

Thanks in advance

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2 Answers 2

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I can honestly say that I've never found a limitation to the number of COM port devices that a machine can use.

I've done a lot of work with machines that use multiple USB to 4-port RS232 devices and I'm pretty sure that on our setups machines quite regularly had COM ports ranging from COM1 upto COM16, all in use at the same time.

I would expect there to be some limitation like 256 ports, but I can't say for sure...

Oh, seems I can, this page for Windows NT (which Win7 derives from) claims 256 ports, but that's a limitation of the WinNT control panel, the actual limitation is being able to address all the ports and is so mind bogglingly huge that it doesn't matter.

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Always great to have someone have actual hands on experience with this kind of stuff. Thanks for the answer. –  Faken Sep 14 '10 at 16:29

The limitation on serial ports on legacy computers was mostly about the limited number of IRQs available and the fact that they only had two UART chips. Even then there were add-on boards that could give you large numbers of ports.

Since the OS and commonly available application software made assumptions based on common hardware, there wasn't very much support for large numbers of ports so that was another limiting factor. However, with drivers supplied with the expansion boards and specialty software that was made for this market, the limitations could be overcome somewhat.

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