[chbot] TTL to Line driver

Marshland Engineering marshland at marshland.co.nz
Mon Sep 21 10:28:17 BST 2020

Finally some time to reply. Something to read if you are bored.

My trusted machine did a wobbly and started machining offset mid job. Each
 time it did this, I lost $400 in parts and 2 hours in down time. Worked all
 day once and and then skipped 2 mm the next. Very frustrating. 

The CNC machine has a South Western Controller made in the US. It is about 35
 years old but an exceptional machine. The position encoder is a wheel that
 runs on the table and its value is compared to the motor encoder to check
 tracking. The wheel encoder disk edge is radiused and the distance movement
 is calibrated by rotating the encoder on its axis. Not the kind of thing you
 can quickly buy from Aliexpress.

Here is one. 


Funnily enough, I did have a spare one of an earlier version I had bought many
 years ago from Ebay. On testing, it did not work. I opened it up and it
 looked very old technology inside. There was the finely graduated rotating
 disk and some odd looking components in it. It looked like there was no power
 going to the pickup circuit. There was rather a big blob of silicone over the
 one part. Grabbed it with a pair of pliers and yanked it off, only to hear
 the bulb hidden inside smash. This is an earlier version of a photo pickup
 with a BULB. I turned down a white LED changed the dropper resistor and I had
 light. I wish I had taken pictures of the 4 photo eyes. They looked like
 pinheads soldered to the PCB board. No bigger than 2mm in diameter. 

After recalibrating the resistors going to each of the eyes to get a nice
 clean signal, it was up and working. 

Whopedo. Unfortunately, the resolution was not the same as my original ones.  

I contacted South Western Industries in the US and we went through a few
 steps. Needless to say, as the fault was intermittent, no fault was found.
 While chatting to the support chap, he mentioned 5 mew m, or something like
 that. Not quite sure what he meant, I questioned him on it.  Looks like was
 saying 5um.  Metric encoder on a US machine ???

Luckily a few weeks before, I had bought a new DRO kit from Aliexpress for my
 bigger lathe. I normally use 1um linear scales however as this is a 1m lathe,
 1um is really small so I opted for a 5um set and saved some $$$. 

There we are, I needed a new 5um linear scale and I had it in stock. A bit of
 drilling and tapping and it was fitted. The TTL output however would not
 interface with the CNC control. I just piggybacked the output of the scale
 onto the encoder I had just repaired and away it went.

I completed my job last Wednesday and shipped it off. Finally caught up on the
 back log of other work today.

I think I may just make a small PCB for the TTL interface with the same driver
 ICS as in the encoder for the new scales. ICs are still for sale in Mouser.  

Hope I haven't put you all to sleep.

Cheers Wallace

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