1 Feb
2025
1 Feb
'25
5:51 p.m.
On Sat, 1 Feb 2025, Steve Lewis wrote:
Reading the async-expansion for the IBM 5110, it talks
about -25 to +25V
(the original spec of RS232?). On the 1980 Color Computer 1, I noticed it
uses -12V to +12V for its RS232. Later in the 1990s, laptops wanted to
sip less less power, and I think RS-232 revisions allowed for as low as -3V
to +3V swings? So those -5/+5V or 3.3 integrations get referred to as
more modern "TTL logic level serial port" (such as generally a USB/serial
adapter) to contrast from prior legacy devices.
I don't think referring to ±5V or ±3.3V as TTL levels is correct by any
means: you can't feed it to any TTL input and expect correct operation.
I'm not sure even whether a TTL circuit would survive supplying negative
voltage; it has been decades since I had a look at how a TTL gate looks
like at the transistor level and I don't remember the details anymore.
Maciej