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Beteiligte Autoren:
Dennis Kuschel, Michael Schoettner

Signal decoder

Startbeitrag von Michael Schoettner am 07.06.2015 07:25

Dear Dennis,

again I have some questions regarding connector PINs, this time related to the V2 connector on the signal decoder PCB.

TDAT = Transfer Data?
Write data to external bus?

OEDAT = Output Enable Data
Read data from external bus?

DDIR = Data Direction
How is this related to TDAT and OEDAT?

This seems to load the bus buffer chip IC28 on Interface and Register PCB. What is the purpose of this signal? And what is the IC28 chip used for?

General question regarding acronyms
What is the “T” standing for (e.g. in TDAT, TADL etc.)?
Transfer, transaction or trigger?

Again many thanks in advance.

Best regards,



Hi Michael,

here are my answers:

TDAT is the internal name for "write data" or "/WR" on the bus connector. This signal goes low when the CPU wants to write data to the external bus, e.g. to a peripheral or to external memory.

OEDAT is asserted low when the CPU wants to read data from the external data bus. It's external name is "/RD_RAM".

DDIR is used to switch the data bus direction, especially the direction of the bus driver IC22. It is low when the CPU wants to write data to the external databus, otherwise it is high. The difference to TDAT is that TDAT is additionally combined with the internal bus clock, so TDAT goes high a half clock period before DDIR goes high again.

IC28 is a pseudo-register that provides a constant value to the internal bus. It is the "constant" register, thus the name OECON. This "register" (it is actually only a bus driver) can provide the values 0x00, 0x01, 0x02 and 0x03. With this trick, for example, the CPU is able to pre-initialize the address bus registers IC40 and IC42 to 0x0003 (address of the interrupt vector in ROM) or 0x0100 (base address of the stack memory in RAM).

"T" is the german abbreviation for "Takt" (in englisch "clock"). It is always a qualifier for write-signals (write something into registers or RAM). Signals that are prefixed with a T are always combined with the internal bus-clock signal.

Best Regards,

von Dennis Kuschel - am 09.06.2015 12:36
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