Connecting to Old Instruments Having +-12V RS232 Voltage Levels

[Pax_Electronica]

Hi,

  I have an old piece of instrument built in the 1980s. It requires +-12V voltage levels for the RS232 interface. My eBay USB to serial dongle doesn't work. Can someone suggest a PCI or PCIe card that can reach +-12V? For example, can NI's PCI-8430/2 do the job? I found the following table in the file http://www.ni.com/pdf/manuals/373170f.pdf, where the "Driver output" is listed to be 5 to 25V. Is this the voltage level of the signal, such as the "Data Treminal Ready" (DTR)?

[K C]

(almost) duplicate post

[Pax_Electronica]

I initially put this post on the Digital IO board. Then I thought this board maybe more appropriate and asked the moderator to change for me (Thanks!).

Anyway, I am puzzled by this "Driver output: 5V-25V" spec. Assuming cable length in zero, i.e. measure right at the port with a scope probe, I should get voltage swings of +-25V?  And the voltage swing would gradually decrease as I add cable length to it? My instrument has its own power. But it needs the incoming signal swing to be 12V, it is not the same as "to be able to work at +-5V".

[K C]

The 5 - 25V spec. is the maximum. Maybe you will measure this at an open line. But normally even with an open line in most cases I saw the voltage will exceed the 12V limit.

You say your device needs 12V is that the specification or ??

Also these old devices need correct input states at the handshake lines. 

Can you show some more information of the device ?

Kees

[Henrik_Volkers]

If you say PCI or PCIe I assume you have a PC (no labtop) to run your application.

Find the documentation of your motherboard, with some luck you find a full servicing RS232 port on the board and only a SUB-D 25/9 connector missing.And maybe you need to activate it in the BIOS. But modern PC often stuck with low level RS232 😞  

For USB a search with USB RS232 12V should point to sources 😉

[Pax_Electronica]

The specification for my instrument says that the DTR pin must be held high at +8V for the interface to be activated. The voltage level is +-12V. That is what the manual says. I have so far ignored this and tried eBay USB to RS232 dongles to no avail. These dongles work well on other RS232 ports.

I have ordered a PCI-8430/2 card, but judging from this thread, it might not work:

https://forums.ni.com/t5/Instrument-Control-GPIB-Serial/RS232-Low-Voltage/td-p/381509

If PCI-8430/2 is the same as PCI-8430/8, then the voltage level will be +-5V.

The only product I can find that addresses this voltage level problem explicitly is the Rainboxes GB-534 RS232 booster that provides +-11V signal levels, for about $250. It is too expensive for me.

[Henrik_Volkers]

Maybe you can ask here:

https://www.small-control.de/USB-RS232-12.html

If you have some USB-RS232 adapters and know where the hot end of a solder iron is, take a look inside.

If you find one with the 'glorious' MAX232, I would try to supply it with an external higher voltage.

A scope always help 🙂

[Henrik_Volkers]

Just had a look at the usual MAX232 spec... 😞

Seems you need something like a

https://datasheets.maximintegrated.com/en/ds/MAX3185.pdf

But still I recommend to look at your motherboard documentation, since it's usually supplied with 12V, chance is still there that you have a full Rs232 on a pin header and all you need is the cable ...

[cstorey]

I haven't looked thoroughly, but you probably want either a PCIe card like

https://www.startech.com/ca/Cards-Adapters/Serial-Cards-Adapters/4-Port-PCI-RS232-Serial-Card~PCI4S9...

or maybe one of these in front of your USB-RS232 cable - https://www.startech.com/ca/Cards-Adapters/Serial-Cards-Adapters/rs-232-to-ttl-signal-converter~IC23...

Haven't tried either, but they look like what you need.  Most USB-RS232 adaptors are now for 1.8/3.3V applications and focus on high speed rather than output voltage levels.

[Pax_Electronica]

The dongles I have now are from eBay and use the CH340 chip. I haven't tried to open the sealed connector yet. I am still hoping to buy a level shifter or a "high voltage level" serial card. If not, then a tested circuit diagram would help too. The instrument was made in Germany, circa 1990. Maybe I will find the original designer in that forum, lol.

The StarTech support told me none of their product supports +-12V

The motherboard serial port on later PCs don't go to 12V either. I wonder what is the time line for the voltage change? Maybe there is a junk yard full of those old PCs.