Do you have a question? Post it now! No Registration Necessary. Now with pictures!
- Posted on
- Vintage Macintosh Quadra 840AV
- Lionel Pinkhard
October 7, 2005, 3:41 pm
rate this thread
I'm looking for a way to connect a VGA monitor to an old Mac Quadra 840AV,
I've built a cable using the pin layout at
home.pacific.net.hk/~johnb/mtv.html, but after connecting it to the Mac I'm
just getting a dead monitor, any ideas? Am I using the wrong cable layout?
I've been struggling the last few months to get the connectors and cable to
make this connector - I can't believe it's so hard to get DB-15 connectors!
Please, any help would be greatly appreciated!
Re: Vintage Macintosh Quadra 840AV
If you haven't already got documentation for the computer, you
can look here. The first problem I had with the PDF file, is it
was not readable on my computer. The fonts were missing in the
document. To fix this, you can use Ghostscript. I used version
8.51 of Ghostscript on a PC. There are conversion scripts in
the "lib" directory of Ghostscript, and a series of Postscript
to PDF conversion scripts. You can also use the Postscript to PDF
scripts, to convert PDF to PDF. The syntax is like this -
"ps2pdf14 in.pdf out.pdf" will redistill the document and restore
the fonts. You execute the command in an MSDOS window on the PC.
You may need to either set up the gs executables in your search
path, or copy the files in the "bin" directory to the "lib"
directory. Place the source file in the "lib" and then the
above syntax should work.
PDF page 62 contains information about the amount of VRAM versus
the monitor resolution and color depth.
If you had not already bought your connectors, this is the
device you need. You could find these in my local computer stores
about 10 years ago. I do not know if these parts would
still be in stock, but this is a very nice little adapter.
"Total Technologies Unimac 82D adapter"
The adapter has tiny switches on it, and the switches are
used to send whatever sense code the user requires.
There is a little more information here, about what the four
synchronization options for the monitor are:
Since you have already bought connectors, you can try building
There is some info here, about the Mac interface:
Note that for some of the extended sense codes, a diode is
used between the sense lines. That is how, when one sense
line is driven low by the computer, the response can be
set up asymmetric.
The first thing to check, is there is a difference between male
and female connectors. It is the pin numbering. The pin numbers
are printed next to each pin, and read 1..8 for the top row
of the male DB-15, but read 8..1 for the female DB-15.
You are making a male DB-15 adapter to female 15 pin VGA.
Looking into the barrel of the connectors, the pin numbers are:
1 2 3 4 5 6 7 8 DB-15 Male
9 10 11 12 13 14 15 looking into barrel
5 4 3 2 1 VGA 15 Pin Female
10 9 8 7 6
15 14 13 12 11
I include the following for reference purposes.
1 2 3 4 5 (The VGA 15 Pin Male on the end of your VGA
6 7 8 9 10 extension cable will look like this.
11 12 13 14 15 The pin numbers work out, when the two
connectors are mated.)
Now, to the specifics of wiring connectors. The R,G,B,H,V
signals are intended for transmission on 75 ohm coax cables.
You have two options. Either keep the wire between connectors
short (1/2") or use coaxial cables. I choose to keep the wires
short, as coaxial cable is a pain to use, especially as the
insulation will melt when you solder. The center conductor of
a coax would go to pins 1,2,3,13,14. The shield of the coax
cable could use the five ground pins 5,6,7,8,10. Notice how
the grounds on 6,7,8 are ideally placed to support coax
cables on pins 1,2,3 if you chose to use coax cable. If you
do not use coaxial cable, and use simple wire, the shorter
the wire, the less fuzzy and jittery the picture will be.
If you see ghosts or blurring in the image on the monitor,
that can be partially due to your wiring job.
Pin # and Signal Name for VGA 15 pin:
1 Red 6 Ground 11 No Connect
2 Green 7 Ground 12 DDC Dat
3 Blue 8 Ground 13 H Sync
4 No Connect 9 No Connect 14 V Sync
5 Ground 10 Ground 15 DDC Clock
DDC_Dat and DDC_Clock are use by a PC, to query the monitor
for resolution information. On the Mac, these are No Connect
and will not be used.
So, what wires are needed ? We need R,G,B,H,V plus all the
five grounds (because the grounds are used on the coax cables
on your VGA cable). In addition, don't forget to connect the
metal shield of the VGA to the metal shield of the Mac DB-15
end, for shield ground continuity. That will give you some
measure of protection from static electricity.
Now, on the Mac end, we have two sets of wires. We have the
R,G,B,H,V and five grounds to go with them. The second set
of wires, is the three sense lines. For some background info,
try reading these two articles.
Nicer looking versions with pictures are here. I had a hard time
finding these, I can tell you! :
Now, if you had stated what kind of monitor you had, I could
have specified more exact options for the sense codes.
Generally, it is best if you have a multisync monitor, as
the monitor will be more tolerant of what comes out of the
Mac, and you are more likely to see some output. For example,
I've used the Multiple Scan 21" sense code for lab use, and
it allowed me to go to the Display control panel, and
set whatever resolution I wanted on a multisync PC monitor.
As stated in HW_30, the three sense lines give 8 possible codes.
To make more codes, later Macs added the ability to drive each
individual sense line, and see if either of the other two
sense lines wiggle. That is what this table is showing:
Table S-2 Extended Sense Line Code Assignments
Sense 2 Low Sense 1 Low Sense 0 Low
Monitor Type 1 & 0 2 & 0 2 & 1
Multiple Scan 21" 10 00 11
Notice that Sense 0 can be grounded here, as it is
always expected to return a 0. In the third column, Sense 0
would be low, when it is driven low by the Mac. Notice
that when Sense 1 is low, Sense 2 goes low. But when
Sense 2 is low, Sense 1 ends up high. This calls for the
use of a switching diode, such as a 1N4148 or 1N4448 type
diode. You can see the use of the diode in the right most
block of figure 4 of hw_30.pdf . Basically, a diode is connected
to Sense 2 and Sense 1. The Anode end goes to Sense 2. The
Cathode end (marked with a black bar on the diode) goes to
Sense 1. Sense 0 can be connected to any one of 5,6,7,8,10
The 1N4448 looks pretty nice - the desirable properties are
a low Vf and a low leakage current:
If you are using a monitor with a fixed resolution, then you
need to find out what that resolution is, and whether the
Mac can make the desired resolution. Then wire up the Sense
signals as required by the info in hw_30.pdf.
1) Hook up R,G,B,H,V
2) Hook up the five grounds for the above five signals
3) Hook up the desired sense code. Multiple Scan 21" is a
good choice for a modern PC monitor. For a really old
fixed res monitor, consult hw_30.pdf. The signals
Sense 2,1,0 either need to be grounded, or need to
be connected to one another in some pattern.
4) Connect the shield of the DB-15 to the shield of
the 15 pin VGA. This can be done with the nuts and
bolts you might used to hold the connectors together.
Soldering the metal would be pretty tough to do with
a small iron, and takes patience and a lot of heat.
Re: Vintage Macintosh Quadra 840AV
> I'm looking for a way to connect a VGA monitor to an old Mac
> Quadra 840AV,
> I've built a cable using the pin layout at
> home.pacific.net.hk/~johnb/mtv.html, but after connecting it
> to the Mac I'm
> just getting a dead monitor, any ideas? Am I using the wrong
> cable layout?
> I've been struggling the last few months to get the connectors
> and cable to
> make this connector - I can't believe it's so hard to get
> DB-15 connectors!
> Please, any help would be greatly appreciated!
> Lionel Pinkhard
Posted using the http://www.hardwareforumz.com interface, at author's request
Articles individually checked for conformance to usenet standards
Visit Topic URL to contact author (reg. req'd). Report abuse: