I have some external equipment in my aircraft that uses 2 RS-232 ports on a laptop (through a PC Card with 2 serial ports). I would like to change the current laptop to a more modern one, e.g. MacBook Pro, and since they don't come with RS-232 ports or PC Cards slots my question is: - Can I expect same functionality if I use USB to RS-232 converters? - Is there any drawbacks using USB instead of RS-232, e.g. protocol, security?
(the equipment are a gyrostabilized camera mount with GPS and with automatic trigger)
USB to RS232 adapter can be bought for a few Euros. They work fine. Usually the data rates of such equipment are very low and there's no problem.
I make these, among other stuff.
You can expect the same functionality as with a real physical COM port - unless the app
doesn't support higher COM port numbers; many older apps are limited to say COM1-COM8 whereas the virtual COM port number of a USB device will be allocated to the first available which could be say COM17, especially on a machine which has had bluetooth COM ports configured... (there is a way to set the USB COM port to a lower value, with certain assumptions)
is doing something dirty e.g. driving the hardware directly; old apps used to do that, in fact anything more than 10 years old might be doing it
is doing something dirty with the modem control lines (RTS,CTS,DTR,DSR,CDI) which many USB-RS232 adaptors do not support properly (one reason we sell ours for £25 - it is very compatible).
most USB-RS232 converters, including ours, have a limited voltage swing on the RS232 e.g. -5V to +5V, whereas the "old" PC RS232 ports usually do -10V to +10V. Both meet the RS232 spec but ... 
USB is distance limited to about 5m whereas RS232 can go much further if the baud rate is not too high e.g. 9600 baud can run some 30-50m, and more with special cables.
Also RS232 can be extended over RS422 to ~1000m but then the modem signals will not work so your RS232 link has to be functional with just TX,RX,GND connected (3 wires in total) and you need to verify that.
Thank you for quick response! I'll give it a try.
I would try to buy a unit which already has the 2 ports in one device. It will usually be less trouble than using two separate converters with an USB hub or similar. In some cases you will have to pay attention where you plug the devices in or the COM numbers will change. Other drivers do not care about that and assign the same COM number regardless in which USB port you plug the device. Another alternative is an ethernet to serial adapter but as they will need an extra power supply this might not be ideal in an airplane.
There are basically two chips in the marketplace, used in these USB to serial converters.
One is from Prolific in China. The other is from FTDI in the UK (made in the USA).
Almost all converters using the Prolific one do not implement the "unique device ID" system and will thus come up as a different COM port each time you move them to a different USB port on the same machine. My USB-232 does this too. The only exception I have used is from Startech - it was about €20.
Some converters using the FTDI chip (and these normally cost more) implement the unique device ID. My USB-485 does that. No matter which USB port you plug it into (even via a hub) it will retain its COM port number. But if you plug two of these into the same machine, you get the two COM ports allright, but you have to get the original driver file and install the drivers again for the 2nd one... (under windows, anyway). I don't do a "USB-232" using the FTDI chip because the bottom fell out of that market...
If you can get a dual FTDI one that would be best. Otherwise, just be careful which USB port you plug these into and make sure they always go back into the same ports otherwise your software will stop working, obviously...
Old-fashioned as I am, I always distrusted USB. Even if I must admit it seems to work fine for the more generic stuff like keyboards and mice, and my own GPS receiver seems to have little trouble at 4800 bd fixed, through a cheapo converter, I would still insist on physical rs232 for things that really matter. But I must admit that is a gut feeling, not supported by any kind of solid research.
Sure enough I could convince my main customer that physical rs232 is a requirement for reliably accessing server's service processors, they happily provided an upper-class laptop to cope with the requirement even in a period of dire budget constraints.
I would still insist on physical rs232 for things that really matter.
I would too.
USB is dead in industrial automation, except in some cases for field config functionality.
For two RS232 ports I would get a PCI RS232 card. But if the computer can't take cards, you can't do it. With some older laptops you could get PCMCIA RS232 adapters.
I would only buy converters that use the FTDI chips (these are the chips I use when I build circuits that need a simple way to talk to USB - they do a great USB fifo IC).
The other thing to watch out for is that older RS232 devices might be expecting the old 12 volt signalling. If you have such a device you'll need to make sure that the converter can do this.
I would only buy converters that use the FTDI chips (these are the chips I use when I build circuits that need a simple way to talk to USB - they do a great USB fifo IC).
Any particular reason for this? I haven't had any problem with Prolific chips either.
FTDI's are interesting if you want to do some nonstandard stuff, like bit-bang I/O.
