[ale] Parallel & serial port cards?

[Alex Carver]

The standard for RS232 is a +15V/-15V signal with an absolute minimum of 
+5V/-5V (not 0V/+5V).  Standard compliant RS232 receivers have to accept 
a very wide range of voltages up to -25V/+25V with a minimum of -3V/+3V.

Cheap USB devices cheat when the 0v/5V signal by relying (hoping) that 
the receiver will treat the zero level as the mark and the 5 V as the 
space.  Good adapters have a chip after the USB-UART bridge chip that 
contains the charge pumps and level shifters needed to produce proper 
RS232 signals.  None of the common USB-UART bridge chips have this 
charge pump on-board.

On 2020-11-15 14:47, Boris Borisov via Ale wrote:
> Well that's from the internet "... Some USB to RS232 converters use
> integrated DC/DC converters to create the appropriate voltage levels for
> the RS232 signals, but in very cheap implementations, the +5 Volt voltage
> is directly used to drive the output."
> 
> On Sun, Nov 15, 2020 at 5:44 PM Boris Borisov <bugyatl at gmail.com> wrote:
> 
>> I think these DAC worked only in real time with precise interrupts to give
>> you constant bitrate aka real time DOS . I doubt there would be +12/-12V on
>> serial port.
>>
>> On Sun, Nov 15, 2020 at 5:02 PM Sam Rakowski <devnull at iamdevnull.info>
>> wrote:
>>
>>> Somewhat related, I've always wondered (it hasn't been an issue for me
>>> *yet*) is whether those USB->RS232 adapters actually swing +/- 12V or they
>>> just stick to TTL levels or similar.
>>>
>>> Also, are there actual USB->parallel port adapters that truly provide a
>>> 'normal' parallel port, with each pin able to be controlled like GPIO pins?
>>> I'd like to make another one of those cheap 'n' dirty R-ladder "DACs" to
>>> enjoy the hi-fi output of the PC speaker for nostalgia's sake.
>>> Sent via BlackBerry
>>>
>>> -----Original Message-----
>>> From: Boris Borisov via Ale <ale at ale.org>
>>> Sender: "Ale" <ale-bounces at ale.org>Date: Sun, 15 Nov 2020 14:54:18
>>> To: Phil Turmel<philip at turmel.org>; Atlanta Linux Enthusiasts<ale at ale.org
>>>>
>>> Reply-To: Boris Borisov <bugyatl at gmail.com>,
>>>   Atlanta Linux Enthusiasts <ale at ale.org>
>>> Subject: Re: [ale] Parallel & serial port cards?
>>>
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> 
> 
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