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A bunch of ZX8302s
Posted: Sun Aug 25, 2024 7:30 am
by RWAP
This is just the top layer of the ZX8302s - not as much variation in these as with the ZX8301s but there are two layers.
Many have the two resistor mod
Has anyone ever discussed what that mod does - I know it was in the service manual - but what does it actually do?
Re: A bunch of ZX8302s
Posted: Sun Aug 25, 2024 12:49 pm
by martyn_hill
Hi Rich!
In my various conversations with others much cleverer than myself, we came to the conclusion that the 'resistor mod' was a means to bias the RAW MDV data lines to the -12v negative voltage rail to overcome the tendency of some MDV ULAs to hold the lines high.
Those lines seem to implement the equivalent of an Open Collector cct (but positively biased) and you can often see a rather sluggish negative/trailing edge to the signal (as compared to a nice, sharp leading, positive edge), especially with multiple drives connected to the MDV bus. The negative bias would therefore help 'shape' those trailing edges more sharply.
That said, I have a single MDV unit fitted to a QL inside a PC case where I had no -12v line available and therefore removed those resistors with absolutely no ill effect (the MDV unit itself had a fresh ULA fitted that you kindly sold me a while back.)
It's only 'safe' when used with one of Sinclairs 'hybrid' digital/analogue 8302 ULAs or the equivalent in the Interface-1 (where a bias down to Gnd is implemented via a pair of 10k resistors instead.)
I asked Charles about whether his Interface-1 ULA replacement (the brilliant vLA-1) could tolerate having such a negative bias added and he said 'no way' as the CPLD/FPGA wouldn't cope well with voltages outside the 0-5v range.
I'll be looking to pick up a couple of spare 8302's from you once available
Re: A bunch of ZX8302s
Posted: Sun Aug 25, 2024 1:54 pm
by Popopo
I would be interesting in a pair too
Just awaiting you name a price for each and where to buy it.
Re: A bunch of ZX8302s
Posted: Sun Aug 25, 2024 8:30 pm
by bwinkel67
They are all available on SellMyRetro (both 8302's and 8301's) for £20 from a friend of his:
Re: A bunch of ZX8302s
Posted: Sun Aug 25, 2024 10:01 pm
by Popopo
bwinkel67 wrote: Sun Aug 25, 2024 8:30 pm
They are all available on SellMyRetro (both 8302's and 8301's) for £20 from a friend of his:
Thank you.
I'll check it out
Re: A bunch of ZX8302s
Posted: Tue Aug 27, 2024 1:55 pm
by RWAP
martyn_hill wrote: Sun Aug 25, 2024 12:49 pm
Hi Rich!
In my various conversations with others much cleverer than myself, we came to the conclusion that the 'resistor mod' was a means to bias the RAW MDV data lines to the -12v negative voltage rail to overcome the tendency of some MDV ULAs to hold the lines high.
Those lines seem to implement the equivalent of an Open Collector cct (but positively biased) and you can often see a rather sluggish negative/trailing edge to the signal (as compared to a nice, sharp leading, positive edge), especially with multiple drives connected to the MDV bus. The negative bias would therefore help 'shape' those trailing edges more sharply.
That said, I have a single MDV unit fitted to a QL inside a PC case where I had no -12v line available and therefore removed those resistors with absolutely no ill effect (the MDV unit itself had a fresh ULA fitted that you kindly sold me a while back.)
It's only 'safe' when used with one of Sinclairs 'hybrid' digital/analogue 8302 ULAs or the equivalent in the Interface-1 (where a bias down to Gnd is implemented via a pair of 10k resistors instead.)
I asked Charles about whether his Interface-1 ULA replacement (the brilliant vLA-1) could tolerate having such a negative bias added and he said 'no way' as the CPLD/FPGA wouldn't cope well with voltages outside the 0-5v range.
I'll be looking to pick up a couple of spare 8302's from you once available
That is interesting - yes I can see how the sluggish trailing edge might cause issues.
Re: A bunch of ZX8302s
Posted: Tue Aug 27, 2024 4:14 pm
by Popopo
RWAP,
I am thinking to buy a QL keyboard membrane from yours and a couple of ZX8302,
Do you think it could be possible to put it all together in order to save money from shipping?
SellMyRetro shows me different sellers.
Re: A bunch of ZX8302s
Posted: Tue Aug 27, 2024 4:37 pm
by RWAP
Popopo wrote: Tue Aug 27, 2024 4:14 pm
RWAP,
I am thinking to buy a QL keyboard membrane from yours and a couple of ZX8302,
Do you think it could be possible to put it all together in order to save money from shipping?
SellMyRetro shows me different sellers.
Yes that would not be a problem - just pm me your details (address and email) and I will send a single invoice for whatever you need.
Rich