[E3-hacking] Another hello

Ian Stirling ian.stirling at mauve.plus.com
Thu Jan 19 16:43:46 GMT 2006

Cliff Lawson wrote:
> Yup, I have to say I was surprised when folks were saying they could connect
> an E3 to a PC without a MAX232?! Both E2 and E3 signals are at TTL levels
> so, by rights, you shouldn't be able to just connect direct and have it work
> as the 16550 in the PC is supposed to be expecting RS232 -/+12 but maybe
> some of them are a bit more "lenient" ? Also, as Andy says, the E3 signals
> are inverted (saved the cost of an inverter on the board!) so we use MAX232
> boxes with a switched 74 series inverter in there too and use it non
> inverted for E2 and inverted for E3. Your mileage may vary.

Back in the dim and distant past, this may have been true.
However, with the growth of low-power devices on the 232 bus, more and 
more stuff has quietly lowered the level at which 0 becomes 1 from 7(?) 
of the MC1488/1489, down to 5V, or even lower.
5V signalling will work just fine with many of todays PCs, even 3.3V on 
quite a lot of them.
Of course, you really need protection, rather than just connecting the 
PC serial port to delicate inputs.
But any 74 buffer chip with a resistor on the 'rx' line from the PC to 
limit the current when it's high should generally work just fine.
Of course, if they are inverted, it working would indeed be kind of odd.

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