RS-232 connections

[ssylee]

I'm puzzle about why the diagram on p. 332 in https://www.electro-tech-online.com/custompdfs/2007/11/mikroc_manual.pdf uses all the RS-232 connections other than pin 9. I had only used TXD, RXD, and ground pins in the past with RS-232 cables. After reading https://en.wikipedia.org/wiki/RS-232, I've come under the impression that the rest of the pins are used in a null modem application, but I'm still unsure. Any insights on this?

 

[dknguyen]

THe null modem thing is partly correct. The rest are used for hardware flow control and handshaking. Hardware flow (and I guess hardware handshaking) control is not always used (just takes more work and doesn't seem to be needed usually). Do I need to explain flow control or handshaking?

On your wiki link, it mentions:
"The handshaking and control lines of the interface are intended for the setup and takedown of a dial-up communication circuit; in particular, the use of handshake lines for flow control is not reliably implemented in many devices. "

 

[ssylee]

If hardware flow control isn't used, do I just connect the transmit and receive pins as well as the ground?

 

[ericgibbs]

I'm puzzle about why the diagram on p. 332 in https://www.electro-tech-online.com/custompdfs/2007/11/mikroc_manual-1.pdf uses all the RS-232 connections other than pin 9. I had only used TXD, RXD, and ground pins in the past with RS-232 cables. After reading https://en.wikipedia.org/wiki/RS-232, I've come under the impression that the rest of the pins are used in a null modem application, but I'm still unsure. Any insights on this?

Hi,
The only connection between the host and remote are the 3 wires TX, RX and 0V.

The wires on the 9Way connector are normally linked as shown, to satisfy the RS232 hardware handshaking lines, required by SOME PC programs.

They are not used as active lines between the host and remote.

EDIT:
The drawing is misleading, IMO its suggesting that an intermediate cable is connected between the remote and the host.
[it shows two 9Way connectors, with three wires coming from the top connector to the PC]

The top 9Way connector actually plugs into the back of the PC, so the handshaking links will most likely be required, depending upon the PC's program.
As the PC's program is written by the author of the drawing, I suspect the links will be required.

Hope this helps.