20mA Current Output from Digital Lines on 6008 USB

[bmkessler]

Hi,

I'm trying to run an old device that requires the ability to source 20 mA on its TTL lines.  I know with a pull-up resistor it is possible to source 8.5 mA from one digital output on the 6008.  One solution would be to use three outputs in parallel to source the required 20 mA or would this damage the 6008?

It seems like the best solution would be to drive a MOSFET off the +5V supply line though.  Anyone have a suggstion for the appropriately spec'ed MOSFET for the job?  Also, is there anything I should be concerned about?

Thanks,

Brian

[Buechsenschuetz]

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>One solution would be to use three outputs in parallel to source the required 20 mA or would this damage the 6008?

It depends... on the output circuitry of the 6008. In general, it is not adviseable to connect logic outputs in parallel. There is some risk that the output with the lowest impedance has to carry most of the current.  

> It seems like the best solution would be to drive a MOSFET off the +5V supply line though.  Anyone have a suggstion for the appropriately spec'ed MOSFET for the job?  Also, is there anything I should be concerned about?

First, you should check whether your peripheral device needs just 20mA at TTL-Level to detect a certain logic state, or whether there are some requirements to detect a different logic state. I.e., some input configurations need a current path to GND to detect a logic state (that's what the generic TTL input needs).

If you just need 20mA flowing into the input, you can do this with a simple pnp-transistor connected to the +5VDC rail. If the input of the peripheral device needs a current path to GND you should consider a true TTL buffer circuit, there are some available with +/- 24mA source/sink outputs.