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ISA card - finding the voltages used

1. Introduction

The design of the ISA bus places a number of voltages in the bus connector. An ISA card may use one, multiple or all of the voltages provided . As a minimum all of the ISA cards use +5V power. Depending on the functions of the card and the way they are realized, the ISA card may use other voltages as well.

This article will give you a practical way of recognizing what voltages an ISA card is using and what is necessary to make the ISA card function on one of the USB2, SSI2, or XPRS families of cards.

2. Voltages on the ISA bus

There are the four voltages provided for use on the ISA bus: +5V , -5V , +12V , -12V. Full details on the ISA bus are available on - Wikipedia, and TechFest.

An ISA card always uses +5V. The ISA card may not use other voltages or it may use one or more of the three other voltages.

The ISA card above has golden pads, as seen at the bottom of the image. Some of the pads are covered in gold some are not. The ISA bus consists of two parts- the 8bit portion (the long part- on the lower left-hand side) and the 16bit portion (the smaller portion- on the lower right-hand side) .

The power supplying pins are located on the 8bit portion - down, starting from the left to right :

pin1 = ground

pin3 = +5V

pin5 = -5V

pin7 = -12V

pin 9 = +12V

pin10 = ground

and the additional voltages to pay attention to are : pin5 = -5V, pin7 = -12V, pin 9 = +12V.

3. Examples

3.1. ISA I/O card - need of +12V and -12V

The ISA card in the picture below is an I/O card with a floppy disk, a hard disk controller, a parallel port and two serial ports.


Going through the pins reveal that pin5 is not connected/used. But pin7 = -12V , and pin9 = +12V are connected/used in this ISA card. There are thick lines going up to the circuit board .

Based on the currents consumed by the ISA card on +12V, -12V, -5V, you can order and use:
- isa-pwr (power supply module) if the consumed currents are less than 50ma
- isa-pwr-xr (external power supply 25W) if the consumed currents are higher, or if unsure on the consumed currents

3.2. ISA network card - no need of additional power

The ISA card below is a network card.


Some of the pads are missing but the position they are located in have to be counted. Pins 5,7 and 9 are all missing.
This ISA card uses only +5V, does not on need an additional power, and may be used directly on USB2, SSI2, XPRS type cards with the power provided through the cables.
Desktop computers may provide high currents, however notebook computers are sensitive to the power use. A specific ISA card may need an external power supply to provide high currents on +5V.

3.3. ISA VGA card - need of -12V

The ISA card on the picture below is a VGA card.


Some of the pads are missing but the position they are in have to be counted. Pins 5 and 9 are missing, but pins7 = -12V is used/connected.

Based on the currents consumed by the ISA card on +12V, -12V, -5V, you can order and use:
- isa-pwr (power supply module) if the consumed currents are less than 50ma
- isa-pwr-xr (external power supply 25W) if the consumed currents are higher, or if unsure on the consumed currents