RS232串口是不是有9针和25针两种？工业上的串口是不是多是25针的呀？

是有9针和25针之分.
但是可以转换,其实是一样的.

PC 串行口定义

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PC/AT 机上的串行口是 9 针公插座，引脚定义为：

Pin Name Dir Description
1 CD Carrier Detect
2 RXD Receive Data
3 TXD Transmit Data
4 DTR Data Terminal Ready
5 GND System Ground
6 DSR Data Set Ready
7 RTS Request to Send
8 CTS Clear to Send
9 RI Ring Indicator

PC/XT 机上的串行口是 25 针公插座，引脚定义为：

Pin Name Dir Description
1 SHIELD - Shield Ground
2 TXD Transmit Data
3 RXD Receive Data
4 RTS Request to Send
5 CTS Clear to Send
6 DSR Data Set Ready
7 GND - System Ground
8 CD Carrier Detect
9 n/c -
10 n/c -
11 n/c -
12 n/c -
13 n/c -
14 n/c -
15 n/c -
16 n/c -
17 n/c -
18 n/c -
19 n/c -
20 DTR Data Terminal Ready
21 n/c -
22 RI Ring Indicator
23 n/c -
24 n/c -
25 n/c -