"GSGeek sympatico.ca>" wrote in message news:wYuta.538$J%5.147813@news20.bellglobal.com... > Hello, > > The 74LS245 is a bidirectionnal bus transceiver with 3-state output. > Depending on the signals present on G* and DIR pins, the chip can let data > bits flow trough its 8 bidirectionnal buffers (G* held high) or disconnect > from the Apple data bus (G* held low). The chip can 'disconnect' by putting > all its ouputs into high impedance. The transceiver is useful to interface > a device chip with bidirectionnal data bits which can't go into high > impendance. > The transceiver is also useful to increase the fanout of data input signals. > The Apple 2 bus' D0..D7 signals can only drive a few TTL inputs (less than > five I believe) per card. > In a nutshell, you won't need one if you have a few chips tied to the data > bus and if each one of them can have their data output signals go into high > impedance (disconnects from the bus). > > Happy hardware hacking - it's fun! > > Yves > > "Glenn Jones" a écrit dans le message news: > Ydtta.106779$kYH.10128@news01.bloor.is.net.cable.rogers.com... > > During the design of an adapter card for the Apple II when is it > > appropriate to use a 74LS245 to buffer the data bus? I see some designs > that > > use this chip and others that don't, so I am not sure if I need to use > one? > > > > Any input would be appreciated. > > > > Thank You > > > > Glenn > > > > > > Yves, I am interfacing an ethernet module to the Apple2 bus. The module has two chips on it...one is a custom chip W3100A and the other is a RTL8201L PHYceiver chip. I am not sure how many TTL inputs this would be equivilent too. The concept is the same as the TFE ethernet board that Adam Dunkels created except that the Wiznet chip is amost 1/4 the price and has a TCP/IP stack built into the 3100A. I have a prototype wired up without the 74LS245 but IIgs hardware reference pg 180 talks about loading and driving rules and this has got me thinking htat maybe I should be using a 74LS245 for the data bus. If you are interested, you can find out more information about my project at http://members.rogers.com/rg.jones/AppleII_ethernet%20_project.htm Thanks Glenn Glenn Jones writes ... > .... > > Yves, I am interfacing an ethernet module to the Apple2 bus. The module has > two chips on it...one is a custom chip W3100A and the other is a RTL8201L > PHYceiver chip. I am not sure how many TTL inputs this would be equivilent > too. > > The concept is the same as the TFE ethernet board that Adam Dunkels created > except that the Wiznet chip is amost 1/4 the price and has a TCP/IP stack > built into the 3100A. > > I have a prototype wired up without the 74LS245 but IIgs hardware reference > pg 180 talks about loading and driving rules and this has got me thinking > htat maybe I should be using a 74LS245 for the data bus. > > If you are interested, you can find out more information about my project at > http://members.rogers.com/rg.jones/AppleII_ethernet%20_project.htm > .... Usually, you would not use a bus transceiver unless there were some reason to isolate the peripheral bus-- like the peripheral (local) data bus is used by a controller on the board to talk with other chips on the board. If you have peripheral board outputs which are seperate from inputs, having an 'LS244 octal tri-state buffer (or a couple of LS365's or LS125's) on the peripheral data outputs is pretty standard practice. It guarantees the peripheral can drive the system bus and can improve peripheral response (when peripheral chip outputs can be tied to 'enabled'). If your peripheral IC's handle data only through a bidirectional bus, you can check IC documentation to see if adding a transceiver or pairs of buffers is a typical option supported by the chip(s) and when adding buffers is recommended. Otherwise, it may be best to leave space for a few extra chips on the prototype and try out the board with no special buffering. Rubywand Rubywand writes: > Usually, you would not use a bus transceiver unless there were some > reason to isolate the peripheral bus-- like the peripheral (local) > data bus is used by a controller on the board to talk with other chips > on the board. I disagree. The Apple bus only supports two LSTTL loads per card on most signals, and only one on some. If your card needs to tie the data bus or address bus to more than two chips, you should have bus buffers, regardless of whether there's any other activity on the local side. Given that suitable data bus buffers (e.g., 74HCT245) are quite inexpensive, there's very little reason NOT to use them. If your address bus lines go to more than one part, I'd recommend buffering them as well.