Dual bus serial comms (RS232 & RS485)

[beyersk]

Hi

I know one gets IC's that can do it in one, but they are generally expensive and sometime not normally stocked by our suppliers making for lon lead time on manuf.

I want to use MAX232 eqv. and say LTC485 to make a dual bus where the user can connect to unit via RS232 or RS485. The problem seems that the RS232 levels will damage the RS485 IC due to over-voltages ootside the absolute max ratings.

Is there an easy way to combine these / overcome the problems ?

Thanks
Beyers

 

[Lewish]

Since all of the previous postings on this topic were deleted, I will re-post my last one.

Explain further what you really want. Do you want a single connector that the user plugs into that can be either RS-232 or RS-485, or what is it that you want the user to do?

 

[beyersk]

Yes, I want combined RS232 and RS485 abilities on one bus, i.e. single connector with only max. 3 wires connected in connector (Gnd, Tx/A (RS232/RS485), RX/B (RS232/RS485) ). Thus the user can plug in with either bus protocol, and the system must preferably check what bus protocol the user is using.

PS. I the mails that was deleted(?), I have given an outline of what possible solutions are (which is more expensive) and how I am doing it now with jumpers (which is cheaper but not under software control or automatic, which is what I want). If anybody is interested in this, let me know and I will repost the sollutions.

 

[Lewish]

Sorry, I don't see a way to do that. The voltage differences will not allow it. For example, if your system was set to RS-232, and the customer plugged in an RS-485 devices, you would either smoke the customers device or the RS-232 drivers in your unit.

 

[beyersk]

Thanks. You are right, some will smoke others won't, but I won't know what the customer's got. So I'll stick with old fashioned jumpers or use the LTC1387 or MAX3160 when plug-n-play weighs more than economics.

 

[IRstuff]

Plug and Play should only be used when there is one and only one function assigned to a given interface. Even if you could get a circuit that worked reliabily, you could not possibly assure that you won't get bad connections or contacts that could give you an erroneous indication of the interface type, causing catastrophic damage to the circuitry.

This is one of the reasons why the USB specification spends so much energy on verifying that the 4 supported connector types cannot be erroneously inserted the wrong way or into an incompatible connector. TTFN