2nd parallel port sometimes not recognized

Hardware questions and modifications

2nd parallel port sometimes not recognized

Postby FortMax » Sun Nov 29, 2020 10:03 am

I have a parallel port sidecar with the LPT2 and bi-directional mods (the Rapport Drive 2 unit has LPT1), but my PCjr doesn't always detect it. I seem to recall a thread where someone mentioned that this is just a thing that happens due to how the system was designed, and that some POKEing will get it to see the 2nd port, but I can't seem to find it.
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Re: 2nd parallel port sometimes not recognized

Postby Brutman » Sun Nov 29, 2020 10:20 am

You are probably referring to an ancient thread that I wrote.

I used to keep a PCjr at my office when I worked for IBM. It had two parallel port sidecars; one for a Zip drive and one for a Xircom adapter. I could use it to telnet to my Linux machine and do my programming on it. It probably made more sense to use the Linux machine directly, and I did most of the time, but having the PCjr as an option to use was fun.

I've never debugged it properly so I can't speculate on the cause. The fix was simple; the hardware was physically there, but the BIOS was not picking it up. When this happens you can run a script that tells DEBUG.COM to force the I/O port address of the second parallel port into the correct spot of the BIOS RAM area. After that you can reboot or just try to use the parallel port directly.

The address of the first LPT port is at 0:408 and it is two bytes. The second slot is at 0:40A. I forget the exact port address to use, but you can look that up. Also, it is possible to manipulate these locations to "swap" the ports if you need to do that for some reason.
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Re: 2nd parallel port sometimes not recognized

Postby FortMax » Sun Nov 29, 2020 11:52 am

So to make sure I get this right, I set 0:40A to 78h, and 0:40B to 02h?

Or am I going at this completely the wrong way?
PCjr w/ Rapport Drive 2 + 384KB Memory Expansion Package, jrIDE
Wanted: light pen
Documentation (including new and updated pages for the Guide to Operations) from:
PC Compact Printer
Speech Attachment (except for the installation instructions)
FortMax
 
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Location: Coppell, TX

Re: 2nd parallel port sometimes not recognized

Postby FortMax » Sun Nov 29, 2020 3:29 pm

OK. Got it working. Thanks.

(now if only I had any idea how to put together an executable for it for convenience's sake)
PCjr w/ Rapport Drive 2 + 384KB Memory Expansion Package, jrIDE
Wanted: light pen
Documentation (including new and updated pages for the Guide to Operations) from:
PC Compact Printer
Speech Attachment (except for the installation instructions)
FortMax
 
Posts: 36
Joined: Fri Feb 07, 2020 9:50 am
Location: Coppell, TX

Re: 2nd parallel port sometimes not recognized

Postby Brutman » Sun Nov 29, 2020 3:50 pm

That part is easy ...

Take what you typed into debug.com and put it in a file. Then run debug.com with that file redirected in from stdin. For example:

debug < fixports.txt


Where fixports.txt has:
e 0:40A 78 02
q

(Or whatever values you typed in ...)
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Re: 2nd parallel port sometimes not recognized

Postby FortMax » Sun Nov 29, 2020 6:19 pm

I meant more like a com file or something.
PCjr w/ Rapport Drive 2 + 384KB Memory Expansion Package, jrIDE
Wanted: light pen
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FortMax
 
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Location: Coppell, TX

Re: 2nd parallel port sometimes not recognized

Postby Brutman » Sun Nov 29, 2020 9:03 pm

Ok, then try this:

Run debug.com and then type the following:

a
xor ax,ax
mov ds,ax
mov ax,0278
mov [040a],ax
int 20
<just press enter here for a blank line>
r cx
c
n setlpt2.com
w
q

When you are done you will have a small (12 byte) program called SETLPT2.COM that sets the address you need in the second slot of the parallel port table.
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Re: 2nd parallel port sometimes not recognized

Postby FortMax » Tue Dec 01, 2020 2:59 pm

Thank you. That was exactly what I was wanting.
PCjr w/ Rapport Drive 2 + 384KB Memory Expansion Package, jrIDE
Wanted: light pen
Documentation (including new and updated pages for the Guide to Operations) from:
PC Compact Printer
Speech Attachment (except for the installation instructions)
FortMax
 
Posts: 36
Joined: Fri Feb 07, 2020 9:50 am
Location: Coppell, TX


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