DIY Ethernet board for IBM PCjr

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I've got this idea 2 years ago (January 2017) after some discussion in IBM PCjr forum. So idea is to make an Ethernet board (with WizNET module) that could be inserted into usually free modem slot of IBM PCjr. And I will need to write some software for it of course...

Original forum discussion from January 2017:

One of the slots (2x15 contacts) of IBM PCjr are intended to be occupied by modem, so we can re-use it, because that modem was useless anyway ( 300 bod : )

This is modem that I also have to measure and copy board dimensions properly:

Existing hole on the back could be used to insert Ethernet jack instead of Phone:

WizNET board could be located this way (upside down):

Schematics of the modem are giving us some ideas how we can connect WizNET module to PCjr:

It's not too many signals there

 A1 - D7         B1 - 5V
 A2 - D6         B2 - /MODEM CS
 A3 - D5         B3 - A9
 A4 - GND        B4 - /TXRES
 A5 - D4         B5 - GND
 A6 - D3         B6 - A2
 A7 - D2         B7 - A1
 A8 - GND        B8 - A0
 A9 - D1         B9 - GND
A10 - D0        B10 - /IOR
A11 - MODEM INT B11 - /IOW
A12 - GND       B12 - GND
A14 - 5V        B14 - TXRES
A15 - 12V       B15 - 5V

It's enough to cover ports 03F8H...03FFH (8 ports total because of 3 address lines) and a single interrupt IRQ4 (that was used for modem on PCjr and it was connected to COM1). Address line A9 is needed to differentiate modem and floppy drive controller because both slots use the same /CS and A9 is the only difference (A9=0 for floppy and A9=1 for modem). Signal /CARD INST should not be grounded in our case, because PCjr BIOS is checking that modem card is inserted using that signal and starting to initialize UART:

WizNet chip W5100 is able to work in so called "indirect mode" when internal 32KB memory are accessible not through full-scale 15-bit address bus, but through 4 registers (addressed by only 2 address lines) and I plan to use exactly this because of lack of address lines in modem slot of IBM PCjr.

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