我在DOS的年代要測試RS-232 COM Port是否正常, 都是用mini tester(如圖):

您可以到電子才料行去買此測試器,上頭的LED可以顯示電氣極性:

'0' = +9 ~ +12V 亮綠色LED,
'1' = -9 ~ -12V 亮紅色LED;

若要在DOS下測試 COM Port,建議可以找 Telix 軟體;
http://www.telix.com/delta/deltacom/tfd/index.html
甚至使用 Copy Text.txt COMx 的指令也可以..(當然還有其他的工具可供測試)

以上僅提供您參考, 希望有幫上您的忙.

參考這篇：http://www.duxcw.com/dcforum/DCForumID3/228.html

There may be a diagnostic program that came with the MODEM, if that is what you are connecting to. You can pretty much tell whether the serial port and cable are working or not by typing something in the Windows HyperTerminal and looking at the LEDs on the MODEM. MS Diagnostics (MSD) that comes with Windows 95 and MS-DOS 6.X is useful for determining if a serial port is responding. A loopback test, which requires a loopback plug/adapter, is a very good test of a serial port. Several commercial diagnostics programs (e.g., AMIDiag: http://www.ami.com/ami/showpress.cfm?PrID=69) come with a loopback plug. When a loopback test is run data is sent out the serial port’s transmit pin to the loopback plug which sends the data back to the port’s receive pin. This test assures the 1488 and 1489 buffer chips or equivalent are working and they are the chips that are usually damaged by a close lightning strike, etc. Scott Mueller’s Upgrading and Repairing PCs has more info and instructions for making a loopback plug. Just search http://www.google.com for the book. Every shop should have a copy. However, the best and easiest to read shop reference on serial ports and cables I know of is Data Communications for Microcomputers by Elizabeth A. Nichols, Joseph C. Nichols, and one of my business acquaintances from the old days, Keith R. Musson, McGraw-Hill 1982. I think it is out of print. An RS-232 breakout box (http://www.bb-elec.com/product.asp?sku=232BOB1E&dept_id=136) and a multimeter are useful tools for troubleshooting serial ports and cables. I test MODEMs by dialing a local Internet Service Provider with Windows Hyperterminal (I used to dial a local bulletin board). Larry

會用烙鐵嗎？通常要找一個9pin或25pin的接頭，把一些腳位用跳線短路，然後執行測試的程式。網路上應該可以找到一堆免費的。因為接腳該怎麼跳跟程式有關係，可以找看看有附跳線圖的。然後就拿烙鐵按圖施工吧。基本上，不管是DOS或是視窗的程式，要測COM埠少不了這個東西。

一般都是用loopback測試COM Port是否正常。