Computer to radio is a standard (?) modem cable wired pretty much as below except with pins2 (9 pin)  going to 2 (25pin) and likewise for pins 3.

From the radio to the controller, only 3 wires go from the 9 pin plug on the controller to the 25 pin on the radio wired as above.(5 - 7 signal ground).  Note: pins 4, 8, and 20 are jumpered together inside the 25 pin connector at the radio.







I believe this is for a NULL modem cable???  (ve6dak - see below)

Serial port interface pinout and signals

9
pin #

25
pin#

Acronym

Full name

Direction

Description

3

2

TxD

Transmit Data

--»

Transmits bytes out of computer or device

2

3

RxD

Receive Data

«--

Receives bytes into computer or device

7

4

RTS

Request To Send

--»

RTS/CTS flow control

8

5

CTS

Clear To Send

«--

RTS/CTS flow control

6

6

DSR

Data Set Ready

«--

This device is ready to communicate

4

20

DTR

Data Terminal Ready

--»

This device is ready to communicate

1

8

DCD

Data Carrier Detect

«--

Modem connected to another

9

22

RI

Ring Indicator

«--

Ringing on telephone line

5

7

SG

Signal Ground

 

 



Note: DCD can be sometimes labeled as CD

Port Pins May Have No Constant Meaning

Only 3 pins of the 9 have a fixed meaning: send, receive and signal ground. This is defined by the hardware interface and you can't change it. But all other pins can be controlled by a software and can be used for other tasks (rarely). EIA/TIA (Electronic Industries Alliance /Telecommunications Industry Association) issued several RS232 standards. RS232 defines both the physical and electrical layers of this interface. RS232 is usually similar with ITU V.24 (pins description and names) and V.28 (electrical). RS232 uses following voltages for signals:

For example, Advanced Serial Port Monitor can automatically control the DTR signal and set it LOW (-12V) when sends data out. A modem (or another device) that receives this DTR signal on another side (if these pins are connected) may do various things. For example, a modem can hang up the telephone line when DTR is LOW. In other cases the modem may not handle this signal or do something else when DTR is LOW (-12V).

It is possible for all the 6 signal wires, but usually in standard devices these signal pins are used as described in the table above. The Advanced Serial Port Monitor software can work with all signal of the RS232 interface.
 

How to connect RS232 ports using cables

A serial port is used to connect devices or computers, and therefore it should be connected to to another serial port. An external device or modem that is connected to the serial port has an identical built-in serial port. It is possible two connection types between serial ports: straight (pins 2 on both sides are connected) or null-modem (pin #2 is connected to pin #3 on another side). First connection type is used between DTE (Data Terminal Equipment) and DCE (Data Communications Equipment) devices (for example, a computer and modem), and the second connection type is used between two DTE devices (for example, two computers or a computer and an external device). There are several cable layouts possible. You can see examples in the "Serial cable connection" chapter.


All signals of the RS232 port is are unidirectional. For example, if pin #2 outputs data, then it is unable to receive any signal using this pin. Therefore, you can't connect two devices that can send data out (DTE-DTE connection) using a straight cable . If you will do it, then they would both send out signals on the same wire but neither would be able to receive any signal.
 

Initially, RS232 ports were used for connecting a dumb terminal (DTE) to a modem (DCE). Nowadays computers used as DTE instead of a terminal. The meaning of the pins are the same on both Data Terminal Equipment and Data Communications Equipment. The names: "receive" and "transmit" should be intepreted from the "point of view" of the computer (DTE).


The serial port was initially intended to be used for connecting DTE to DCE using a straight-thru (null-modem) cable.