I hope someone can help answer some questions for me.
I am designing a circuit that will require me to build a parallel bus fan out-in arrangement that will have up to 17 elements on a bi-directional bus. I plan to use pull up resistors to keep the bus logic HI during the time the bus elements are tri-stated.
1. My specific question is as to the value of the bus pull-up resistors to use. I do not want them to overpower the receiver/driver chips ability to sink current to get a valid logic low on the bus leads. Even though the bus will have up to 17 elements attached, it will only have 2 active at any time 1 send and 1 receive). Transceiver chips I am planning to use are either 74ACT245 or 74AC245 with a 5VDC source. Would the resistor value of 4.7K ohms be OK and work on either chip listed above, or should the value be higher?
2. Second question is more general in nature. Can anyone tell me how many elements the 74ACT or 74AC chips are capable of driving? I have heard that generally speaking, CMOS can drive around 50 and TTL around 10. So, is the 74ACT variety consideed a CMOS or TTL chip in this regard?
Some of ths chips I plan to use are 74xx04, 74xx08, 74xx138, 74xx245, and 74xx573 all of which will be either ACT or AC variety due to the processing speed.
Thanks
Dialtone
I am designing a circuit that will require me to build a parallel bus fan out-in arrangement that will have up to 17 elements on a bi-directional bus. I plan to use pull up resistors to keep the bus logic HI during the time the bus elements are tri-stated.
1. My specific question is as to the value of the bus pull-up resistors to use. I do not want them to overpower the receiver/driver chips ability to sink current to get a valid logic low on the bus leads. Even though the bus will have up to 17 elements attached, it will only have 2 active at any time 1 send and 1 receive). Transceiver chips I am planning to use are either 74ACT245 or 74AC245 with a 5VDC source. Would the resistor value of 4.7K ohms be OK and work on either chip listed above, or should the value be higher?
2. Second question is more general in nature. Can anyone tell me how many elements the 74ACT or 74AC chips are capable of driving? I have heard that generally speaking, CMOS can drive around 50 and TTL around 10. So, is the 74ACT variety consideed a CMOS or TTL chip in this regard?
Some of ths chips I plan to use are 74xx04, 74xx08, 74xx138, 74xx245, and 74xx573 all of which will be either ACT or AC variety due to the processing speed.
Thanks
Dialtone