Zilog SIO/2 programming

[Jon Jones]

I’m playing with writing my own code to work with the Zilog SIO/2. Things are going (surprisingly!) well but I’ve spotted something confusing in SIO data sheet.

In the Reading Registers section (page 312), it says:

"To read the contents of a selected read register other than RRD, the system
program must first write the pointer byte to WR0”

But I can’t find any other mention of RRD. Is this a typo of RR0? If so, after every read does the SIO “reset” back to register 0?

Jon.

[Mark Durham]

From my limited exposure to the SIO, all the registers are either RRn or WRn where n is a digit. If your datasheet is hard to read or possibly OCR'd then there's a good chance that the digit 0 became a D.

The SIO technical manual says:

Following a read or write to any register (except WR0), the pointer will point to WR0.

From memory you have to set the pointer up again in WR0 after every read or write, unless you are accessing RR0 or WR0.

Hopefully I've got that right!

Mark.

[Alan Cox]

The original documents appear to have been OCR'd to make the modern ones. On the old documents it looks more like a 0.

The WR/RR thing is a complete pain in the arse when you have interrupts involved and need to read anything but RR0.

Alan

[Jon Jones]

I didn’t realise many of the docs had been OCDed. Someone’s done a great job of tidying them up! This is what I’m seeing:

[Mark Durham]

I would be wary of any documentation not from the original manufacturer that looks that pristine for a part of that age.

However I must admit that it does make searching the document a lot easier if the original has been OCR'd.

[Ed Porter]

Introducing errors into the text is not doing a great job of tidying
them up. The 1994_Zilog_Z80_Microprocessor_Family_Users_Manual.pdf on
bitsavers includes the SIO docs, is searchable, and isn't mangled.

https://bitsavers.org/components/zilog/z80/1994_Zilog_Z80_Microprocessor_Family_Users_Manual.pdf

SIO starts around page 400.

-ed

[Alan Cox]

The OCR and clean up documents are from Zilog. The original documents were 1970s era and presumably at some point Zilog redid them and OCR imported them into more modern tools.

Zilog manuals in general contain minor errors though, like most vendors.

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