ISA PNP

8/5/2000

ISA PNP -- The Industry Standard Association (ISA) Bus was defined in the early days of personal computing. It is essentially the IBM designed 16 bit bus introduced in the PC-AT. Early ISA bus cards either hardwired the device port, interrupt (IRQ), and DMA addresses or allowed them to be varied with physical jumper wires. In the early 1990s "jumperless" ISA cards started to appear whose configuration could be programmed and, in some cases saved permanently in Non-Volatile RAM. Some worked from drivers loaded in Config.sys or Autoexec.bat. Others were configured once by special DOS software with the settings being retained in NVRAM.

In 1993 and 1994, Microsoft and Intel started developing Plug and Play. Although the primary focus of PNP was on PCI devices, a specification was also promulgated for ISA bus cards.

The process for setting up ISA PNP cards is fairly complicated. Basically, there are two fixed ports that ISA PNP cards monitor -- 279 and A79. The PNP software uses them to start the initialization process and assign a third port at some otherwise unused address in the range 0203 to 3FF. The cards are put into programming mode, isolated, interrogated individually, and assigned resources based on the card's requests and the resources available.

While simple enough in theory, this is subject to the "usual" PNP problems. Defective cards may lock the machine. Adding new cards can alter the configuration of existing cards which confuses OSes other than Windows 9/2000. User controls, if present at all, are poorly designed, confusing, and probably incomplete.

"PNP BIOSes" may offer no or limited ISA PNP support and few, if any, offer useful user support for non-Windows OSes.

As of 2000, the situation seems to be that there are at least four kinds of "jumperless" ISA cards. There are cards that are designed to be explicitly configured on each boot, but do not expect or support plug and play. There are cards that expect to be programmed once and to save their configuration until reprogrammed. There are cards that are intended to support ISA Plug and Play but do not work exactly in compliance with the specification. And there are cards that fully support Plug and Play. A few cards support multiple jumperless modes -- e.g. can be either PNP or "permanently configured". In addition, the degree of ISA support in any given BIOS is apparently questionable.

Configuring jumperless ISA cards is generally done by trial and error.

Microsoft's solution to this problem is to eliminate the ISA bus -- a process they term "innovation".

Return To Index Copyright 1994-2002 by Donald Kenney.