Re: [N8VEM-S100:5596] S100 Serial IO Design Question

["G. Beat" <gregor...@gmail.com>]

RS-232 (serial communications) can be grouped into 3 typical implementation types.

1.) 3-wire usage.  RxD, TxD, and GND (dara lines).  Basic connection, easiest for USB conversion.

2.) 5-wire usage. RxD, TxD, GND (data), plus RTS, CTS (control lines) for Hardware Flow Control.
Schematic of typical FTDI FT232R cable (USB to TTL conversion chip) for 5-wire (UART/TTL) to USB.
https://www.sparkfun.com/datasheets/DevTools/FTDI%20Cable%205V.pdf

Beauty of the FT232R series is that serial lines (UART/TTIL) can be inverted via software configuration (EEPROM),
without additional hardware.

3.) 9-wire (AT-style support).  1984 implemenation of RS-232 with DE-9F connector.
Largely fallen out of favor as Dial-Up (POTS) modems (that used RI, ring indicator line)
 disappeared at beginning of 21st century (or last 10 years).

4.) Custom Variations.  Software Applications and custom hardware devices have been known to use the 6 serial control lines (RTS, CTS, DTR, DSR, DCD, RI) 
for other non-standard purposes (signaling or control).
SOME implementations of the PIC, ATmega, Arduino boards use the DTR/DSR lines for slave processor signaling (ready, reset, etc.).

G. Beat
chicago