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View Full Version : Looking for Tarbell Cassette Interface Schematic



ab0tj
August 9th, 2017, 03:31 PM
Does anyone have a good copy of the schematic for the Tarbell cassette interface? I'm trying to recapture it in KiCad so I can make a PCB for a quasi-replica. I have found a couple PDF copies of the manual online, but they are scanned at fairly low resolution making it hard to make out some parts of the schematic.

Thanks!

Chuck(G)
August 9th, 2017, 04:18 PM
Isn't it in the manual (http://www.classiccmp.org/dunfield/s100c/misc/tb1001.pdf)? Pages 19 and 20. It doesn't include the power supply part (regulators), but otherwise is pretty complete.

ab0tj
August 9th, 2017, 04:31 PM
Sure, I've been working from the manual so far, but some parts aren't very readable. It will probably be good enough to get the job done if nothing else exists. Thanks.

Marty
August 9th, 2017, 04:45 PM
Hi All;

Ab0tj, there's one in the S-100 handbook, along with a host of other schematics..

THANK YOU Marty

1944GPW
August 9th, 2017, 05:10 PM
Have you had a look through old issues of Kilobaud magazine and BYTE on archive.org?
Searching 'tarbell' in a few 1977 issues I spotted
Meet the Tarbell/KC Interface Kilobaud April 1977 p44
Circuit Kilobaud February 1977 p128
Cassette I/O Format Kilobaud August 1977 p18
Profile of Don Tarbell Kilobaud May 1977 p139
etc.
...lots more there, for the digging. They are a goldmine.

Al Kossow
August 9th, 2017, 05:33 PM
Circuit Kilobaud February 1977 p128


it is the circuit for the "Kill A Baud" PE 2400
fun to re-read that article about the problems with all the formats though

1944GPW
August 9th, 2017, 06:44 PM
it is the circuit for the "Kill A Baud" PE 2400
fun to re-read that article about the problems with all the formats though

Fair enough, but can you explain this bit: "This article and a following one will tell you how to build your own PE 2400 interface. While basically intended for NRZ recorders, phase encoding can be used on audio recorders if the baud rate is kept high enough. A hobbyist cassette board using it is marketed as the Tarbell interface. Several other interface kits also can use it."

Al Kossow
August 9th, 2017, 07:50 PM
Fair enough, but can you explain this bit: "This article and a following one will tell you how to build your own PE 2400 interface. While basically intended for NRZ recorders, phase encoding can be used on audio recorders if the baud rate is kept high enough. A hobbyist cassette board using it is marketed as the Tarbell interface. Several other interface kits also can use it."

Kilobaud was trying to make their own standard which didn't have the problems they perceived the others to have, which is what they describe in the articles.
Tarbell apparently used synchronous rather than asynchronous transmission, which required the use of the Intel 8251 serial chip, PE 2400 also used phase encoding but with asynchronous character encoding, 1 stop bit and even parity.
They also didn't like checksums.

Al Kossow
August 10th, 2017, 06:32 AM
Does anyone have a good copy of the schematic for the Tarbell cassette interface?

I found my manual this morning. PM me your email adr and i'll send the two schematic pages to you.
I'll upload the whole manual to bitsavers later this week

ab0tj
August 10th, 2017, 08:00 AM
Thanks all for the recommendations, guys. :D I ordered a used copy of the S100 Bus Handbook (should have one anyway), now I will be reading through lots of issues of Kilobaud magazine, and Al probably has a better scanner than whoever scanned the other manual found online :rofl: - That one has been good enough to get started but has also led to a lot of questions like "Is that an AND or a NAND gate?" "Is there a junction there?" "Is that blob a 5 or a 6?" etc etc.

Al Kossow
August 10th, 2017, 11:12 AM
the whole manual is up now
http://bitsavers.trailing-edge.com/pdf/tarbell/Tarbell_Cassette_Interface_Jan77.pdf

ab0tj
August 11th, 2017, 12:30 PM
Hi Al,

If I could bother you again just once more... On page 12 of the PDF, is the smudge at the bottom of the schematic for the modification present on your hardcopy as well? I assume that resistor there is just acting as a pullup for the last two pins of the 74L30? I'd like to make my implementation of this board jumper selectable between the two tape formats.

Thanks,
Alex

Al Kossow
August 11th, 2017, 12:40 PM
Hi Al,

If I could bother you again just once more... On page 12 of the PDF, is the smudge at the bottom of the schematic for the modification present on your hardcopy as well? I assume that resistor there is just acting as a pullup for the last two pins of the 74L30? I'd like to make my implementation of this board jumper selectable between the two tape formats.

Thanks,
Alex

it it missing on the original

i think it's pretty safe to say it is pulling up the unused inputs

ab0tj
August 11th, 2017, 12:42 PM
Ok. Thank you for checking.

ab0tj
August 18th, 2017, 07:39 AM
Anyone bored and want to check over my schematic? It doesn't match the one in the manual exactly but as far as I can tell it should work as is. I changed a couple of parts to avoid hard to find* parts where possible, but was not able to find suitable replacements for the 8T20 or 7496 chips. I also added Tarbell's modifications to (hopefully) make the board switchable between the Tarbell and Kansas City standards, and added a relay to support operating the tape deck's Remote input.

Corrections and suggestions are welcome.

https://drive.google.com/file/d/0B86v5EfF8_PdMTRZejAzRHFIMzQ/view?usp=sharing

*As in, I typed the part number into the Jameco website and it didn't come up. All of the parts were still found after a short eBay search, although at vintage pricing.

Al Kossow
August 18th, 2017, 07:58 AM
Anyone bored and want to check over my schematic? It doesn't match the one in the manual exactly but as far as I can tell it should work as is. I changed a couple of parts to avoid hard to find parts where possible, but was not able to find suitable replacements for the 8T20 or 7496 chips. I also added Tarbell's modifications to (hopefully) make the board switchable between the Tarbell and Kansas City standards. I also added a relay to support operating the tape deck's Remote input.

Corrections and suggestions are welcome.

I suppose it wouldn't be as much fun, but couldn't you just do this with a microcontroller and maybe an op-amp on the input for gain control?

if not, there are hundreds of 8T20s available here
www.ebay.com/itm/221607581167

you can replace the 5 bit shift registers with functionally equivalent 8 bit ones, and get rid of the 7474s at the end of the chain

ab0tj
August 18th, 2017, 08:58 AM
I suppose it wouldn't be as much fun, but couldn't you just do this with a microcontroller and maybe an op-amp on the input for gain control?

if not, there are hundreds of 8T20s available here
www.ebay.com/itm/221607581167

you can replace the 5 bit shift registers with functionally equivalent 8 bit ones, and get rid of the 7474s at the end of the chain

Agreed, a microcontroller and a small amount of glue logic would do the job just as well, perhaps even better with the right programming. That project would be fun in its own way but also doesn't have quite the same vintage appeal to it...
I did already secure some 8T20 and 74LS96 chips, but I'd like to make this board available to the s100computers group if there is interest, so I'll look into using commonly available 8-bit shift registers. One less part for other builders to have to track down on eBay. Thanks for the pointers.