2m542 source or sink current help?

[irving2008]

Hi jazz,

The machine is a momus design belt driven, il post pics ASAP. Motors are 381oz in from keling inc. I will check the parallel cable thanks, that is one I've overlooked. Looking at the circuit of the bob the output current (74hc244d schmitts) can only source just over 7mA current and the optical isolators on the drivers expect up to 16mA so I think it's definitely out of spec there and needs to be addressed, I think I have a solution using logic level mosfets to sink the optical isolator GND on dir step and ena so I will try today and report back. There is probably a few things going on, always the way so far, I may have too much preload on the bearings too.

The drive current could be the reason for missed steps. M542 is 7mA min, 10mA typical, assuming 5v supply and 270ohm resistor as per leadshine datasheet. But the driver can only output around 3.8v so assuming a 2v LED you're actually getting something like 6mA. Having said that, you should never drive optos from a logic output, you should always sink to ground, i.e. the driver PUL+ to +v, usually an isolated 5v supply, driver PUL- to BOB output, configure as active low. The hc244d can sink 7ma nominal with 0.4v drop so a sink will drive the LED harder. you'll get nearer 8-9mA.

Also likely your parallel port is 3.3v out and BOB is expecting 5v signals (the 'logic high' for a 244d on 5v supply is 3.15v). too much preload is stalling the motors is a distant possibility...