darry wrote on 2022-08-31, 05:24:
I always believed that USB to RS-232 adapters that come equipped with a DB9 connector are able to handle and outout RS-232 levels, presumably by integrating something like a MAX232 to handle conversion to/from the USB adapter's TTL inputs/outputs .
Hm, I think it depends on the type of USB/RS-232 converter.
The cheap ones with a DE-9 connector you can find on eBay are 12v tolerant, but don't generate plus/minus 12 volts.
That's how they differ from the other types, the USB TTL or USB TTL converters as used by Arduino people. They must not be operated past 5v (or 3,3v nowadays).
That being said, there used to be high quality USB Serial converters, like the Keyspan USA-19HS and its sisters.
These are intelligent, programmable converters. They also worked on Mac OS 9 and could emulate the old Mac ports (to some degree).
The problem is, that the RS232 has 8 I/O lines, but the MAX232 charge pump can only handle 2 signals simultanously. RXD and TXD, in our case, I assume.
"It has two drivers and two receivers. Which can be used at the same time for the conversion of data between two different RS232 and TTL/CMOS devices."
Source: https://microcontrollerslab.com/max232-ic-pin … iption-example/
Also, the MAX232 is almost as big as the whole USB Serial adapter itself.
- To be fair, back in the 1980s, it was tiny compared to a Z80 SIO..
Fun fact: Some people used the LM7805 voltage regulator as a step-down converter from RS232.
The 7805 was connected directly to the TXD pin (RS232 side); thus whenever a +12v level was there, it was converted to 5v.
This was a poor man's approach for TTL/RS232 conversion.
On the TTL side, a diode (optional) connected to the TXD pin to RXD (RS232 side).
The RS232 was tolerating a +5v signal as a +12v signal (circa +3v was the minimum).
Edit: The same could have been done with a dozen diodes in series, I assume.
Also, it's perhaps safer to add a few extra resistors for current limiting,
despite the fact that the real RS232 (16550 UARTs) already has current limiting and protective diodes built-in. 😉
Another cool thing: The MAX232 was also used as a cheap voltage supply for +12v/-12v.
More precisely, to generate positive/negative voltages needed for opamps (operational amplifier), like the 741.
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