IBM 5170  -  Motherboard Switch/Jumper Settings

In the move from the 5160 to the 5170, IBM moved the vast majority of motherboard configuration into what IBM refers to as SETUP
Remaining is one configuration switch, a configuration jumper, and a shunt block on type 1 motherboards.

Switch SW1

This switch is used for selecting the video card type (MDA, CGA, EGA).

Card/s Fitted Required SW1 Setting      Comment
MDA only Rear position If in the wrong position, "501-CRT Error" may be seen on screen at power-on.
CGA only Front position If in the wrong position, "401-CRT Error" may be seen on screen at power-on.
EGA only Either position  
MDA + CGA Rear: MDA is to be the primary of the two
Front: CGA is to be the primary of the two
Info source: Page 3-26 of IBM document here

5170_sw1.jpg The switch is located adjacent
to the power connectors.

Pictured in the rear position (towards rear of computer).

Jumper J18

This jumper controls whether the 5170 motherboard decodes 512 KB of RAM, or 256 KB of RAM.

'512 KB' is the normal setting.

The only reason for changing the jumper to the alternate position is when all of the following is true:
* You have a type 1 motherboard (has piggybacked RAM chips); and
* That motherboard has only RAM bank 0 populated - provides RAM from 0 to 256 KB address range; and
* You are going to provide the additional 256 KB (addresses 256 to 512 KB) via a RAM card (instead of using motherboard bank 1).

5170_j18.jpg The jumper is located at the front of the

Pictured in the '512 KB' position.

Capacitor C26

The end of a small screwdriver fits into the slot on the top of the adjustable capacitor.

When using a composite monitor (or TV) as the display, adjusting the capacitor fine tunes the colours that appear. Adjusting the capacitor does not affect the colours that appear on a digital monitor (MDA/CGA/EGA).

Technical: Fine tunes the motherboard 14.31818 MHz clock signal, which in turn, fine tunes the NTSC 3.579545 MHz colour burst signal that the CGA card sends to the NTSC composite monitor.  (Diagram)


Shunt Block U131

Shunt block U131 is only found on type 1 motherboards. On later motherboards, U131 is hardwired.

Set according to the type of ROM used in sockets U17/U27/U37/U47: Either type 27128 or type 27256.
'27256' is the normal setting, because the IBM supplied BIOS ROMS in sockets U27 and U47 are of type 27256.

Diagrams related to U131 are here and here.

5170_u131.jpg Shown in the normal setting
(for 27256 ROMs).