User Port Pins

Modding and Technical Issues

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cbmeeks
Vic 20 Enthusiast
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User Port Pins

Post by cbmeeks »

I posted this over on comp.sys.cbm so sorry for the cross-post.

I'm curious about how to access certain pins of the C64 User Port. Going by the pinout here:

https://www.c64-wiki.com/wiki/User_Port

I see there are some pins I'm not familiar with.

Code: Select all

4       CNT1       Counter 1, from CIA #1 
5       SP1        Serial Port 1, from CIA #1 
6       CNT2       Counter 2, from CIA #2 
7       SP2        Serial port 2, from CIA #2 
8       /PC2       Handshaking line, from CIA #2 
9       ATN        Serial attention in 
... 
B       /FLAG2     Flag 2 
... 
M       PA2        PA2 
I have a basic idea what the counters are for but I'm looking for more detailed information on all of those pins.

Are there any programming and/or schematics out there that indicate how to use them in various projects?

Thanks
Cat; the other white meat.
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Mike
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Re: User Port Pins

Post by Mike »

A sharp look into the schematics of VIC-20 and C64 is surely instructive to clear this up.

You have some pins connected to the +5 VDC and 9 VAC supply voltage, and there's also a /RESET signal line. For the "TTL level" RS232 as implemented by the KERNAL, both computers use a defined set of the port pins and handshake lines to connect to the other end (be it another computer, terminal, printer, modem, whatever).

Other than that, it's a *user* port: you can use the user port edge connector as equivalent to the port pins of the respective VIA or CIA chip. That means, in any way you want - you just have to honour those are TTL/NMOS signals, capable of driving only a certain type of load. Certain hardware drive speeders use this to implement an 8-bit-parallel protocol with the disk drive. For another example, I use 4 of those port bits to provide an extra 4 address lines for the banked colour RAM of my VFLI mod.

That one was much easier done than adding another dedicated chip with an own I/O port.
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