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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, one configuration jumper, and a shunt block on type 1 motherboards.

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Switch SW1

This switch is set according to the type of video card fitted (MDA, CGA, EGA).

Card/s Fitted	Required SW1 Setting	Comment
MDA only	Rear position	If in the wrong position, known symptoms are:   • "501-CRT Error" may be seen on screen at power-on.   • See the 'No progress past ...' section at here.
CGA only	Front position	If in the wrong position, known symptoms are:   • "401-CRT Error" may be seen on screen at power-on.
EGA only	Either position
VGA 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

The switch is located adjacent to the power connectors. Pictured in the rear position (towards rear of computer).

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Jumper Block J18

This jumper block 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).

The jumper block is located at the front of the motherboard. Jumper pictured in the '512 KB' position.

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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)

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Shunt Block U131

A shunt block at position U131 is found on type 1 motherboards.  On later motherboards, U131 is hardwired (photo).

The shunt block has two possible settings (see photo below).  To change the setting, the shunt block is removed from its socket, horizontally reoriented 180 degrees, then refitted.

Set according to the type of ROM used in sockets U17/U27/U37/U47: Either type 27128 or type 27256.

On very early type 1 motherboards, four BIOS ROM's of type 27128 are fitted, and accordingly, U131 is set to the '27128' setting.
Later, two BIOS ROM's of type 27256 (fitted in sockets U27 and U47) were used instead, and accordingly, U131 is set to the '27256' setting.

Diagrams related to U131 are here and here.

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Connectors J19 and J20