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So I'm trying to revive an old (late 90's) noise logger (Acoustic Research Labs EL-215 for those familiar) and I've been looking into USB-RS232 connectors. I have port settings from the original documentation which are as follows:

  • EIA RS-232-C
  • 1200 - 19200 Baud
  • 8 Data Bits
  • 2 Stop Bits
  • Hardware Flow Control
  • (note: parity isn't specified)

I have an old Belkin F5U109 adapter, which hasn't worked so far - so I'm trying to work out how the Belkin is different to other USB-RS232 adapters. I also read that Prolific PL2303 and FTDI seem to be the leading USB-RS232 chipsets that nearly all modern USB-RS232 adapters seems to use.

What features should I look for in an adapter to give me the best chance of making it work with my hardware? Whats the main difference between the Prolific and FTDI chipsets?

I don't care which operating system I need to use as I'm proficient in Linux and can easily spin up a VM with VirtualBox or qemu if need be. Hardware uses DOS software, so could also run dosbox if required. Worst case I can reverse engineer the whole thing and write my own program to communicate with the EL-215, but I'd rather avoid that!

Any help much appreciated!

Edit: Here's what I've tried so far That link seems to suggest that software flow control (XON/XOFF) doesn't work. I've investigated the device in trying to connect to and found that DTR and DSR are not connected, but RTS and CTS are. When I run the DOS software (through Windows XP on a VirtualBox VM, Belkin drivers installed, COM1 8,2, hardware flow control, no parity) which is supposed to connect with the device, I get connection error - it times out waiting for response from device.

I put the multimeter on the pins of the Belkin while using the DOS software. DTR goes from -9v to +3v momentarily, as does RTS. Obviously DTR is ignored by the device because its not connected, so RTS going high should trigger a CTS response from the device but it doesn't.

So I thought that the Belkin is perhaps waiting for DSR to go high before doing anything, so I bridged DTR to DSR, but still no response. I found it strange that DTR only momentarily goes high as if properly implemented it should stay high for the entire duration of the connection.

It's either the Belkin logic levels are not high enough (I think RS232 needs >3v to trigger) or it incorrectly implements hardware flow control. Belkin information about the adapter refers to connecting a PDA so maybe its a specific implementation for those devices...?

I have ordered FTDI and Prolific PL2303 adapters in the hopes that they might work better. Backup plan is to build a circuit to control RS232 pins individually with like an Arduino or something.

Craig Skipsey
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  • Bizarrely, someone had a recent [question](https://unix.stackexchange.com/a/659785/119298) on the Belkin F5U409, which had the same vendorid:deviceid as the F5U109 (design dating from 2003 it seems). The Linux driver does not do software flow control, but might support hardware flow control. What have you tried and what didn't work? – meuh Jul 29 '21 at 08:36
  • See edit above re: what I've tried so far... – Craig Skipsey Jul 30 '21 at 22:05
  • Yes, 3V is the absolute lowest input value for RS232, but 5V should be the minimum output value, so you are probably right that the adaptor is limited in its application. I wonder how the baud rate is negociated though, should it get that far. – meuh Jul 31 '21 at 09:30
  • Did you have any luck? Personally, for old hardware like that, F5U109 adapters never work for me. Prolific sometimes work, but I've got two PL2303-based adapters here which act differently (hardware XON/XOFF on chip works fine on the one by Kensington, but seems to be disabled on the generic branded one). Did your FTDI fix your problem? I almost ordered one on Amazon, but when I zoomed in on the image that said "GENUINE FTDI", I could actually see that the chip said Prolific on it. – hackerb9 Aug 20 '22 at 03:21
  • Yeah I did get it to work, but got reasons unrelated to the RS232 dongle. 1. Used a null modem cable 2. Used a really slow speed like 2400 or something After making these changes, it didn't seem to matter which dongle I used as they all worked. Another lesson that overlooking simple stuff can lead going down a rabbit hole! – Craig Skipsey Aug 21 '22 at 11:40

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