Modem

Greetings, oh venerable Evgeny!

10-Nov-98 15:23 Evgeny Milun sent a letter to Kirill Chubenko:

KS>>> There is a problem here, the fact is that the modem only installs on COM4
KS>>> IRQ3 or 4 What to do with this?

KC>> Then you need to slightly rework the diagram. How, I don’t know, presumably
KC>> to transfer several wires.

EM> Exactly. As far as I remember now - A8, A9 and A10.

No. Here, I wrote a small FAQ for changing the connection scheme
(by Mikhail Kondratiev) for the modem to work on different COM ports.

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■ The COM1 adapter decodes ports 3f8H through 3ffH
■ The COM2 adapter decodes ports 2f8H through 2ffH
■ The COM3 adapter decodes ports 3e8H through 3efH
■ The COM4 adapter decodes ports 2e8H through 2efH

In the “ISA lamella” column, the letter “A” before the number means
that the contacts are located on the “front” side of the board, where
all elements installed. The count of the 1st contact is from the “hardware”, where the connectors
for connecting to the telephone line are located.)

┌──────────┬────┬────┬─── ─┬────┬────┬────┬────┐
│ISA-lamella│ A22│ A23│ A24│ A25│ A26│ A27│ A28│
├──────────┼────┼────┼─── ─┼────┼────┼────┼────┤
│CPU address │(A9)│(A8)│(A7)│(A6)│(A5)│(A4)│(A3)│; address bus 80x86
├──────────┼────┼────┼─── ─┼────┼────┼────┼────┤
│ COM1 │ 1 │ 1 │ 1 │ 1 │ 1 │ 1 │ 1 │ <-default
├──────────┼────┼────┼─── ─┼────┼────┼────┼────┤
│ COM2 │ 1 │ 0 │ 1 │ 1 │ 1 │ 1 │ 1 │
├──────────┼────┼────┼─── ─┼────┼────┼────┼────┤
│ COM3 │ 1 │ 1 │ 1 │ 1 │ 1 │ 0 │ 1 │
├──────────┼────┼────┼─── ─┼────┼────┼────┼────┤
│ COM4 │ 1 │ 0 │ 1 │ 1 │ 1 │ 0 │ 1 │
└──────────┴────┴────┴────┴────┴ ────┴────┴────┘

Accordingly: "1" - apply logic to this lamella of the ISA connector."1"
(preferably through a resistor, if at +5V)
"0" - apply logic to this lamella of the ISA connector."0"
(connect to the "ground", "common" wire)

And here is how the interrupt levels are distributed, in depending
on the selected COM port. (In PC, of ​​course):

■ COM1 forces interrupt level 4 (IRQ 4 is handled by INT 0cH vector)
■ COM2 forces interrupt level 3 (IRQ 3 is handled by INT 0bH vector)
■ COM3 shares IRQ4. BIOS checks the Int ID regs to see who rank
■ COM4 shares IRQ3. BIOS checks the Int ID regs to see who rank

The numbers of the ISA slot lamellas responsible for a specific interrupt:

(The letter “B” means that the contacts are on the “reverse”
side of the board, where the elements are notare installed. The count of the 1st contact is from the hardware, where the connectors for connecting to the telephone line are located.)

IRQ3 - B25
IRQ4 - B24; according to the MK scheme - by default.
IRQ5 - B23
IRQ6 - B22
IRQ7 - B21

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Sincerely, Felix !