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Old 14th Oct 2018, 6:49 pm   #34
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Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
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Default Re: 486 PC. Keyboard problem.

Originally Posted by buggies View Post
Assuming the serial printer works (tested on another pc?) then you could send text strings to it from MSDOS then plug it into the other ports to check them.

Without loading a terminal program, the MSDOS "mode" command will show which ports are present e.g.


Status for device COM1:
Baud: 1200
Parity: None
Data Bits: 7
Stop Bits: 1
Timeout: OFF
CTS handshaking: OFF
DSR handshaking: OFF
DSR sensitivity: OFF
DTR circuit: ON
RTS circuit: ON

then to send a string to that port:-
echo 1234567890 > com1:
I had another play this afternoon. At this stage I just want to prove that the serial ports are working before I attempt to connect to the machine.

At start up the BIOS shows three com ports with addresses $3F8, $2F8 and $3E8.

Entering "mode" at the DOS prompt was a dismal failure, possibly due to the fact I can't find a way of pausing the display before it scrolls off the screen. I couldn't find a way of generating the pipe character | from the keyboard I was using.

Entering "mode com1/status" at the prompt returns "RETRY=NONE". Same for com2 and com3.

I configured com port 1 using "mode com1:9600,N,8,1,P. Did the same for com2 and com3. Got responses showing ports had been configured.

mode com1/status now returns "RETRY=B" Same for com2 and com3.

Entering "echo 1234567890 > com1" at the prompt returns "invalid device request writing com1". Same for com2 and com3.

I assume I have a hardware problem with the com ports?
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