Just a quick question here... I've noticed when looking at some 74LS139 chips, some of them say "Dual 2 line to 4 line Decoder/Demultiplexer" while some instead are called "Dual 1 of 4 Decoder/Demultiplexer". But both are 74LS139s, and have the same pinouts and from what I can see the same function.
For example:
T74LS139M1 chip -
http://www.datasheetarchive.com/T74LS139M1-datasheet.htmlVS
74LS139A -
http://pdf1.alldatasheet.com/datasheet-pdf/view/27996/TI/74LS139.htmlJust curious because my local store has a bunch of the T74LS139M1 that they are trying to clear out, and I've used the 2-4 line 74ls139s before and had no issues with decoding a SNES game with them.
If the full part number is the same, it's the same. At least with the 74 series parts, they're usually structured something like vendor74technologypartpackage
"2 line to 4 line" is because they have two address lines. "1 of 4" is because each half can only select one at a time.
I have a FF6 cart here, which uses a SN74LS139N to select between the 3 rom chips. That appears to be a 1 of 4 decoder version (according to the datasheet i Googled).
So, for the purpose of using one of these decoders to select between 3 or 4 flash chips in a SNES cart should work regardless of whether its a 2-4 line or a 1-4 line, right?
Ignore the description; look at the part number. First, remove the letters. If the remaining numbers don't match, then it probably won't work. If they do match, examine the letters in the middle of numbers, e.g. LS, HC. If they match, then it'll probably work. In some cases nonmatching letters are compatible.
A '139 chip is a decoder with 2 inputs and 4 outputs. The 2-bit input is a binary representation (00, 01, 10, 11), while the 4-bit output is what is known as "one-hot" because only 1 output is ever active at once (active-low, so 1110, 1101, 1011, 0111). Because of that, you select "one of the four outputs" or you could also say that the "two inputs are decoded to four outputs", hence "one OF four" or "two TO four" are the same thing.
Also, the difference between the letters in the middle (e.g. LS/HC/HCT/etc) describes the technology family (e.g. TTL/CMOS/etc.). The SNES uses TTL logic, so the ones used in SNES carts are LS. However, a more compatible alternative, in case you wanted to replace the ROM/RAM chips with CMOS parts, would be the HCT family, which has TTL-compatible inputs and CMOS outputs, which follows the design paradigm of permissible inputs and strict outputs.
74**139 is a standard catalog part, 54**139 is military-grade. I'm not really sure what the difference between CD74xxx and SN74xxx is...
Quote:
The SNES uses TTL logic, so the ones used in SNES carts are LS.
This is probably getting out of topic, but why do SNES carts use TTL chips which were already obsolete in the early '90s, instead of using HC chips, like the NES carts does ?
I'm pretty sure the SNES processor is CMOS.
qwertymodo wrote:
I'm not really sure what the difference between CD74xxx and SN74xxx is...
I apparently said it poorly above, but in the 74xxx and 40xx parts, the bit before the 74 or 40 usually indicates the manufacturer. (CD = TI; SN = formerly Motorola, now ON Semiconductor; DM = Fairchild; BA = Rohm)
Bregalad wrote:
This is probably getting out of topic, but why do SNES carts use TTL chips which were already obsolete in the early '90s, instead of using HC chips, like the NES carts does ?
HC parts at least used to be slower than TTL parts. I don't know if that was true during the SNES era.
lidnariq wrote:
qwertymodo wrote:
I'm not really sure what the difference between CD74xxx and SN74xxx is...
I apparently said it poorly above, but in the 74xxx and 40xx parts, the bit before the 74 or 40 usually indicates the manufacturer. (CD = TI; SN = formerly Motorola, now ON Semiconductor; DM = Fairchild; BA = Rohm)
Nope, I just managed to miss it as I was reading through the thread. My bad
What confused me though was that TI sells both
SN74** parts and
CD74** parts. Maybe SN was National Semi, and TI kept the part numbers after the merger?
qwertymodo wrote:
Nope, I just managed to miss it as I was reading through the thread. My bad
What confused me though was that TI sells both
SN74** parts and
CD74** parts. Maybe SN was National Semi, and TI kept the part numbers after the merger?
And sometimes they just copied the part numbers from another manufacturer altogether, including the initial letters.
http://en.wikipedia.org/wiki/wikipedia:7400_series#Part_numbering_scheme wrote:
First, although sometimes omitted, a two or three letter prefix which indicates the manufacturer of the device (e.g. SN for Texas Instruments, DM for National Semiconductor) although these codes are no longer closely associated with a single manufacturer, for example Fairchild Semiconductor manufactures parts with MM and DM prefixes, and none.