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I am working on a project where I am reusing a VMEbus backplane to create a bus between some cards of my own design. This backplane has passive termination on both ends.

I am trying to find a buffer that would be appropriate for driving some VMEbus signals in either push/pull or open collector/drain.

VMEbus termination is quite "heavy", using 330 over 470 ohm resistors on both ends, which to my mind acts more like 165 over 235 ohms, or about 30mA pull up and 20mA pull down? This, I think, exceeds what 74HC etc series logic can do. 74LVC seems to have a maximum capability of +/-50mA so would in theory work.

I think I might be able to get away with a couple of 74LVC1G125s, but I'm just not sure, and hoping someone might be able to have a second glance over my thoughts and let me know if I'm on the right track?

Some examples of signals I need to drive are a clock line in push/pull, and a reset line as open collector/drain.

For driving the address and data busses I have spec'd in some 74FCT transceivers.

Thanks

Tom S
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  • Are you using 5 V, 3.3 V, or some other logic level? CMOS or TTL (or something else)? – The Photon Apr 11 '23 at 15:15
  • 5V is the only voltage rail that I am using. Most of the devices I am using are CMOS, but have TTL compatible inputs. Where a device requires CMOS level inputs I am being careful to drive them with CMOS level outputs. – Tom S Apr 11 '23 at 15:22
  • The VMEbus specification (rev C.1, 1985) calls for both "standard" drivers that can sink 48 mA and "high current" drivers that can sink 64 mA. If the bus supply voltage is 5 V, the termination at each end of each line has a Thévenin equivalent of about 3 V and 200 Ω. – Dave Tweed Apr 12 '23 at 10:57

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VME bus! The VME design was built in an era where processors ran at much lower speeds; the Motorola 68000 was running at 1 MHz clock speed, and I believe a 2 MHz version came out later. If you try to push this bus at today's speeds, you will need a big power supply.

If you are using 74FCT you must be running 3.3 volt logic. The issue is that the high current capability of these parts are used to support fast switching times and the parts are not really rated to provide continuous high output currents on all eight channels at the same time. So pay attention to the maximum output current for the device; this must be divided between the outputs. Also, when you see the "maximum" current capability for an output, this is often the current into a short circuit, so the rule of thumb is to divide this number by two.

John Birckhead
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  • You might be thinking of the 6800. The 68000 had a minimum clock requirement of 4MHz (I believe to maintain internal dynamic registers). But Im not using 3.3V logic, its all 5V. FCT series logic operates off 5V but outputs 3.3V to reduce switching times, but all of my devices where concerned have TTL compatible inputs, so this is fine. My project is using a 68000 but at (initially) 10MHz. Im not intending to try and push this thing into 10's or 100's of MHz - no way! – Tom S Apr 11 '23 at 16:48
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    My experience was from 1988, so I trust yours! – John Birckhead Apr 11 '23 at 16:52
  • The VMEbus is basically a buffered version of the 68000 bus. The 680x0 family had clock speeds in the range of 8 to 33 MHz. The bus itself is asynchronous, but it can do block transfers at a rate of up to about 16 Mwords/second, which is 64 MB/s when using 32-bit data. Back in the day, I was designing Apollo engineering workstations. The earlier ones were based on VMEbus, and even when the CPUs switched to a synchronous bus, we included VMEbus for user expansion. – Dave Tweed Apr 12 '23 at 11:11