Most "clone" VGAs have only IRQ 2/9 as option (even the 16bit cards), but usually it can be deactivated.

Many old programs and games are badly programmed when it comes to sound support. Other notorius examples are IRQ7 for SB or 0x220 for the GUS base address. Of course the soundcard manufacturers are also to blame (Creative really messed up things when they changed the standard setting from IRQ 7 to 5).

You can tell if an ISA card allows you to change the IRQs by checking the pins and the connections. If the pins that correspond to IRQ3-7 are missing or the traces don't connect to anything, then you know that the board must use IRQ2.

Perhaps David Wolf may be an exceptional case that it does not use the midi port in such a way that it triggers an IRQ.

(Creative really messed up things when they changed the standard setting from IRQ 7 to 5).

Actually,they aren't really to blame as the IRQs were freely switchable on the Sound Blaster. Sound Blasters can use IRQs 2, 3, 5, 7 and Sound Blaster Pros can use IRQs 3, 5, 7, 10, and all can be done through jumpers. Its up to the programmers to ensure that the user can select the right settings.

Remember that Creative designed the Sound Blaster to work on IBM PCs and 8-bit XT buses. Available IRQs were few and far between, and using a default of IRQ7 was preferable to IRQ5 (which was used by the XT Hard Drive Adapters.)

I was using this pinout chart:
http://www.interfacebus.com/Design_PCAT_Card.html

I have checked all IRQs on my: scc-1, lapc-i, sb2.0, sbpro2.0 to make sure i understand it correctly (everything was connected properly - following lines from pin to jumpers 😀 ).

Paradise PVGA1A-JK (8-bit, 256kB) - doesn't use any IRQs
Cirrus Logic CL-GD5422 (16-bit, 1MB) - doesn't use any IRQs
UMC UM487F (8-bit, LPT, I think CGA, I don't have a 9-pin monitor to check) - uses only IRQ7 (for printer I guess)
Trident TVGA9000B (16-bit, 512kB) - uses only IRQ2 (I also have a manual for this card, it has only a few jumpers and they have nothing to do with IRQ)
Tseng ET4000 (16-bit, 1MB) - uses only IRQ2
Ahead V5000 (16-bit, 1MB) - uses only IRQ2

I think David Wolf uses LAPC's IRQ2 cause it doesn't play music when I change LAPC's IRQ from 2 to something else (I hear only single note from time to time (very rarely).

Does it mean IRQ2 is not really used by any graphic card and can be freely used by any other device? I guess this is the reason why Roland used it as default in its mpu-401 cards.

So IRQs will be used this way:
2 EGA/MPU-401
3 COM2
4 COM1
5 Fixed Disk
6 Diskette
7 LPT1, SB

Great Hierophant: So you can add one more device for IRQ3 😀 What would it be? 😉

IRQ3 just so happens to be the default for the IMFC... 😉

IBM's Enhanced Graphics Adapter uses IRQ2, even though other cards with similar or better capabilities may not. You could find out very easily if this could be a conflict by trying some software with that uses an MPU-401 interface at the same time.

I am trying to ascertain whether IBM's PS/2 Display Adapter also uses an IRQ and whether it can co-exist with a monochrome (Hercules) graphics card like the EGA can. If both are true, then my recommendation would change as it would be fully functional (EGA requiring a daughterboard and RAM upgrades), not require a rare and expensive 5154 TTL monitor and use a regular VGA monitor.

IRQ3 just so happens to be the default for the IMFC... Wink

Could it be that the "midi interface" is merely a serial port masquerading as such?

This article clears up the matter to a real extent:
http://www.osdever.net/tutorials/irqs.php?the_id=37

It states that some EGA cards use IRQ2 to avoid snow using the interrupt to allow programers to write to vertical retrace. The programmer can restrict writes to Video RAM to only when the EGA triggers an interrupt during vertical retrace. This is one way to solve the CGA snow problem, which required the programmer to monitor bit 3 of port 3DAh or write through the BIOS. I also believe that the reason for this is because both IBM's CGA and EGA used single-ported DRAM modules, which don't allow for reads (from the graphics controller circuitry) and writes (from the processor) at the same time. I believe that the IBM VGA card may also use similar DRAM, so it would also use IRQ2 for the same purpose.

Also, the article says that a printer port doesn't need to use an IRQ for unidirectional transfer. IBM didn't use a bidirectional printer port until the PS/2. IBM's design basically keeps the printer port as an output port with some status lines. As the board itself is a simple interface, no device I know of would trigger an IRQ.

On closer examination, it may be possible to use a video card and a midi card that both are set to IRQ2. In IBM's VGA tech ref, it says that you can share the interrupt with another device. The VGA should fire off an interrupt during vertical retrace, allowing the program to write to the video RAM without having to worry about visible snow. At the same time, another VGA register will tell that a vertical trace interrupt is pending.

Enabling vertical retrace interrupts is the programmer's responsibility, so when a programmer writes his interrupt handler code, he can detect whether the interrupt came from the VGA or not. If not, then the interrupt must be from the midi controller, so the program should not write to the screen at that time. If so, then he should proceed with the write. Presumably, any program that allowed you to use EGA with an MPU-401 interface would have had to taken account of this when they wrote the interrupt handler. IBM's EGA and VGA cards did not allow you to change the interrupts (especially as there weren't any that hadn't been taken by their hardware.) Neither did Roland's early MPU-401 interfaces.

A new standard of almost perfection has been achieved, thanks to my perusal of the XT Technical Reference Manual.

The last model of the IBM PC/XT supports 640KB of RAM on the motherboard without any alteration (not that the earlier XT boards require any serious modification to support 640KB.) This eliminates the need for a conventional memory expansion card. With the IBM PC Expansion Unit, a full 16 ISA slots are available to the machine, if only 14 are useable.

05/09/86 BIOS
IBM PC Math Coprocessor (8087-5)

IBM Expansion Unit Extender Card .

IBM Color/Graphics Display Adapter + IBM Color Display (5153)
IBM Diskette Drive Adapter + 2 x IBM 5.25" 320/360KB Diskette Drive + 1 x IBM 3.5" 720KB Extenal Diskette Drive
Hercules Graphics Adapter + IBM Monochrome Display (5151) +
IBM CPS Matrix Printer (5152)
IBM Game Control Adapter + 2 x IBM PCjr. Joystick (custom adapter)
IBM Printer Adapter + IBM Color Graphics Printer (5182)
IBM Music Feature Card + Midi Adapter Unit
IBM Asychronous Communications Adapter + Microsoft Serial Mouse

In the main unit, we have almost everything from IBM. I have tried to keep all essential devices within the main unit. One slot is used to connect to the Expansion Unit. The Asych card is connected to slot 8 because that slot is very finicky about the cards it accepts. The Diskette Adapter can support two internal disk drives and two external disk drives, but IBM only allowed for one external disk drive. The disk drives are full-height models. Ideally, the midi adapter unit should come with the Music Feature Card. Finally, because IBM never made joysticks for the PC, I have decided to use PCjr. joysticks with a simple pin converter/adapter.

IBM Expansion Unit Receiver Card
IBM Fixed Disk Adapter (Xebec 1210) + 2 x 10MB IBM Fixed Disk Drives
Roland MIF-IPC + Roland MPU-401 Midi Processing Unit + Roland MT-32 Multi-Timbre Sound Module
Adlib Music Synthesizer Card
Creative Music System Card (a.k.a. Creative Game Blaster)
Covox Sound Master + 2 x digital pads
Creative Sound Blaster .5
IBM Asychronous Communications Adapter + Hayes Smartmodem

IBM Model M Keyboard (1391401).

In this part of the case, we have two 10MB full height drives. The Fixed Disk Adapter can support two of these drives. The Roland interface card is a simple device and the MPU-401 is a big box with the controller circuitry and the MT-32 is attached to it. The Hayes Smartmodem is of the external variety, and varieties exist in 300, 1200 and 2400 baud.
The Covox should take care of any Speech Thing games as well as early Covox titles. The Sound Blaster .5 is my term for a Sound Blaster with the CMS and Adlib chips removed.

I had a 5150 once.. it really is god awful slow.. like wait for keystrokes to be echoed to the screen slow. Maybe with a 286 board it might be tolerable though.

My vision has really changed this year, since I have discovered that fantastic machine known as the Tandy 1000 SX. It is my 4.77MHz machine of choice for the following reasons:

1. Five 8-bit ISA expansion slots
2. Comes with a printer port and two joystick ports
3. Can be upgraded to 640KB of RAM with standard RAM
4. 4.77MHz and 7.16MHz switchable
5. Tandy Video and Audio
6. Superb CGA compatibility and composite video
7. Supports 360KB and 720KB floppies
8. Bugfixes incorporated.

Sure the joystick and printer ports are physically non-standard, but they are compatible. The keyboard is its only real problem. The SX is just as fast as an equilavent XT, just a little less memory is available to it.

The above posts of mine have a problem with them, they are trying to make a 4.77MHz PC run software that it does not do very well. A 4.77MHz machine is good for running booters, BASIC games, converted PCjr. cartridge games, text & CGA games, and the like. It is not really meant to run EGA or VGA games, and even Tandy graphics are a stretch for the processor. 512KB is the maximum amount of memory that any game should be using on such a machine. The games should fit on no more than 2x720KB floppies. Its sound chip is about the most advanced sound device that the appropriate games would use, but keeping an Adlib or a Game Blaster around wouldn't hurt.

What about other Tandy 1000s? The TX, TL, SL, and RL are not 8088 based. The EX/HX only has one/two floppy drives and very limited expansion capabilities. Their keyboards are attached to the computer, so you can't replace it if the system goes bad. Their power supplies are anemic, even for Tandys. They don't have standard slots, and adapters are no longer available. If you can find a fully upgraded upgraded HX, get it. The 1000/A/HD does not allow you to disable the onboard peripherals, does not allow using other cards, does not have a fast speed, and comes with less slots.