address lines and data lines

[bluejay]

Before you say anything, keep in mind that I am still learning electronics. Every piece of knowledge like this helps me.

https://www.atariage.com/2600/archives/schematics/Schematic_2600A_Low.html

I was looking at the schematics of a 2600 so I could hack around with it and realized a handful of pins on each ic were labeled "address lines" and "data lines" and aren't connected to anything, on the schematics at least. At first I thought; "well where the hell am I supposed to connect them to? They must lead to something!" Second, I thought, "well what the hell are these lines anyway?" So here I am, asking to you lot on AtariAge. I made a hazardous guess that I am supposed to connect each address and data lines to their matching pins on each ic and cartridge connector, but that still doesn't answer what they are. So what are data lines and what do they connect to? Thanks in advance:)

[+DrVenkman]

Data lines in a computer system are exactly what they sound like - they carry data bits (0’s and 1’s, represented by +5V and 0V in a typical vintage system) from one place to another inside the computer. The actual physical lines on the paper for data and address bus lines are often omitted in printed schematics because they can make the schematic too cluttered to read. An 8-bit system like the 2600 has 8 data lines (usually denoted D0 - D7) and up to 16 address lines (D0 - 15 usually). The 2600 actually uses a pin-reduced variant of the 6502 called the 6507. The 6507 has only 13 address lines, with the top 3 bits being dropped. So the address bus lines are denoted A0 - A12 on the 2600. 

 

Bottom line, any chip or component with a signal line denoted with “Dx” (where “x” is the number 0 - 7) indicates that the component should be considered connected to that particular data line. Same thing with components or pins connected to “Ax” (with “x” being 0 - 12) - those indicate the address bus lines.  Again, the schematic drawing itself may or may not include lines on the paper for each of these but they may also just be omitted, with a label as to which signal is on the pin or connected to that component. 

 

Physically, in the running system, each address or data line will have a voltage on it, either 0 or +5V, depending on what is going on inside the system. Probing at them with a logic probe or oscilloscope will show the signals pulsing up and down irregularly corresponding to the information flowing around the system. Using a digital multimeter will not usually help here because the signals are cycling up and down at different speeds and rates depending on the code running in the system - most (cheap-ish) meters will give you a weird averaged voltage if there’s a signal going up and down irregularly on the pin so they don’t really help when looking at address and data lines. You really need at least a logic probe to know whether what’s happening on any given address or data line is expected. Better would be an oscilloscope so you can see the activity on the a screen over time. Better still would be a full logic analyzer but for a <$100 vintage game system, a several thousand dollar logic analyzer is way overkill.

[bluejay]

1 hour ago, DrVenkman said:

Data lines in a computer system are exactly what they sound like - they carry data bits (0’s and 1’s, represented by +5V and 0V in a typical vintage system) from one place to another inside the computer. The actual physical lines on the paper for data and address bus lines are often omitted in printed schematics because they can make the schematic too cluttered to read. An 8-bit system like the 2600 has 8 data lines (usually denoted D0 - D7) and up to 16 address lines (D0 - 15 usually). The 2600 actually uses a pin-reduced variant of the 6502 called the 6507. The 6507 has only 13 address lines, with the top 3 bits being dropped. So the address bus lines are denoted A0 - A12 on the 2600. 

 

Bottom line, any chip or component with a signal line denoted with “Dx” (where “x” is the number 0 - 7) indicates that the component should be considered connected to that particular data line. Same thing with components or pins connected to “Ax” (with “x” being 0 - 12) - those indicate the address bus lines.  Again, the schematic drawing itself may or may not include lines on the paper for each of these but they may also just be omitted, with a label as to which signal is on the pin or connected to that component. 

 

Physically, in the running system, each address or data line will have a voltage on it, either 0 or +5V, depending on what is going on inside the system. Probing at them with a logic probe or oscilloscope will show the signals pulsing up and down irregularly corresponding to the information flowing around the system. Using a digital multimeter will not usually help here because the signals are cycling up and down at different speeds and rates depending on the code running in the system - most (cheap-ish) meters will give you a weird averaged voltage if there’s a signal going up and down irregularly on the pin so they don’t really help when looking at address and data lines. You really need at least a logic probe to know whether what’s happening on any given address or data line is expected. Better would be an oscilloscope so you can see the activity on the a screen over time. Better still would be a full logic analyzer but for a <$100 vintage game system, a several thousand dollar logic analyzer is way overkill.

Yay, I can finally finish my watered-down mini Atari 2600 board!

[+Stephen Moss]

You may know this already, but to add to what DrVenkman said Data lines can be bidirectional and so used to both send and receive data, where as Address lines are unidirectional.

In the 2600 as in any computer system the microprocessor (6502) is in control, is sets the Addresses of memory locations it wants to either read or write data to and sets the R/W line to indicate if data is to be read to written from the selected device (RIOT, TIA or cartridge).

 

In more complex computer systems the upper address lines are also fed into an Address decoder the output of which controls the chip select pins of various IC's to select if memory or a certain peripheral is to be accessed, only one of which will be active at any time as determined by the status of the upper address lines. In the 2600 there is no Address decoder and I believe the RIOT/TIA chip select is performed more directly dependant on the status of Address pins A7, A9 & A12.  

[bluejay]

6 hours ago, Stephen Moss said:

You may know this already, but to add to what DrVenkman said Data lines can be bidirectional and so used to both send and receive data, where as Address lines are unidirectional.

In the 2600 as in any computer system the microprocessor (6502) is in control, is sets the Addresses of memory locations it wants to either read or write data to and sets the R/W line to indicate if data is to be read to written from the selected device (RIOT, TIA or cartridge).

 

In more complex computer systems the upper address lines are also fed into an Address decoder the output of which controls the chip select pins of various IC's to select if memory or a certain peripheral is to be accessed, only one of which will be active at any time as determined by the status of the upper address lines. In the 2600 there is no Address decoder and I believe the RIOT/TIA chip select is performed more directly dependant on the status of Address pins A7, A9 & A12.  

Yikes, I gotta read this over again later:)