Sharing peripherals

General discussions related to the Altair 8800 Clone

Sharing peripherals

Postby mail@gabrielegan.com » March 27th, 2015, 11:22 am

I've got a standalone paper-tape punch that I'd like to share between three VDU terminals connected to an Altair running Multi-User BASIC. Does anybody know what is the best way to wire this up? My idea is that 'saving' is done by LISTing a BASIC program to the VDU but diverting the output (or copying the output) to the paper-tape punch (much like an ASR-33 Teletype can punch what is being listed to its printer). I know I can get RS-232 switch boxes and try to be quick enough to divert the signal going from the Altair to the VDU to instead go from the Altair to the paper-tape punch. But is there a device for splitting an RS-232 signal so I don't have to make the manual switch? Ebay has some cables that seem to split an RS-232 signal but they don't look like they have any electronics in them. There are also RS-232 splitter boxes that seem to have electronics in.

Or is there a better way to share this peripheral?

Regards

Gabriel Egan
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Re: Sharing peripherals

Postby AltairClone » March 29th, 2015, 8:24 am

With only transmit and ground wired through, a passive splitter can be used to have multiple receivers listen to a single sender. However, multiple senders to a single receiver (as in your application) requires an active splitter. These can be as simple as diodes to wire-or transmit signals.

Wired-or requires that only one transmitter be transmitting at any given time or garbage results at the receiver. this means you'll have to require all users to stop working while one person loads or saves to tape.

Do the terminals you are using have a pass-thru port that can be turned on and off manually or through a control sequence?

Mike
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Re: Sharing peripherals; and optimizing Timesharing BASIC

Postby mail@gabrielegan.com » June 4th, 2015, 2:57 pm

Sorry, I meant to reply to this: no, the terminals don't have spare ports that I know how to control. In fact, I've now tried this setup with manual switching boxes and it' works pretty well. Students don't mind waiting whenever someone wants to save: they find the paper-tape punch very cool. (And I agree!).

I want also to have the students use Timesharing BASIC. I've succeeded in booting this from a paper-tape containing DBL v3.5. (You don't supply the bootstrap code as Supplementary Material, Mike, but I was able to transcribe the octal numbers you call out in the video and they work fine!) Could I just check: if I use the control panel to switch out all the PROMs (don't need 'em when using DBL v3.5 on paper tape I believe) then I'll have more RAM for the users with this BASIC, right? I want to give each of three users as much RAM as possible.

Last question: is the reason you set the sense switches to 0000 when using your Teletype as a console and as loading device because it uses two stop bits? I'm using sense-switch settings of 0001, which means one stop bit, right? (I've read all the documentation I can find and didn't come across this distinction.)

Regards

Gabriel
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Re: Sharing peripherals

Postby AltairClone » June 4th, 2015, 5:07 pm

Gabriel,

Glad to hear your progress! You can refer to the "Loading and Initializing BASIC" section of the 1977 BASIC manual http://altairclone.com/downloads/manuals/BASIC%20Manual%2077.pdf. Here you'll find the bootloader referenced in the DBL paper tape readme file ("same bootstrap loader you'd choose to load Extended BASIC version 4.x"), which is also the same bootloader I entered in the video.

In this section you'll also see information about setting the front panel switches A15-A12 for BASIC and A11-A8 for the checksum loader. Yes, 0000 is 2SIO 2 stop bits and 0001 is 2SIO 1 stop bit. When booting Timesharing BASIC, you'll want to set A11-A8 to zero (well, at least A11). Normal disk BASIC doesn't look at A11-A8, but Timesharing BASIC does.

Turning off the PROMs will free more RAM for BASIC as you surmised.

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Re: Sharing peripherals; and optimizing Timesharing BASIC

Postby toml_12953 » June 5th, 2015, 2:11 am

mail@gabrielegan.com wrote:Sorry, I meant to reply to this: no, the terminals don't have spare ports that I know how to control. In fact, I've now tried this setup with manual switching boxes and it' works pretty well. Students don't mind waiting whenever someone wants to save: they find the paper-tape punch very cool. (And I agree!).

I want also to have the students use Timesharing BASIC. I've succeeded in booting this from a paper-tape containing DBL v3.5. (You don't supply the bootstrap code as Supplementary Material, Mike, but I was able to transcribe the octal numbers you call out in the video and they work fine!) Could I just check: if I use the control panel to switch out all the PROMs (don't need 'em when using DBL v3.5 on paper tape I believe) then I'll have more RAM for the users with this BASIC, right? I want to give each of three users as much RAM as possible.

Last question: is the reason you set the sense switches to 0000 when using your Teletype as a console and as loading device because it uses two stop bits? I'm using sense-switch settings of 0001, which means one stop bit, right? (I've read all the documentation I can find and didn't come across this distinction.)

Regards

Gabriel

If you have the PROMs (anyone using a Clone does), why not switch on the Turnkey module and jump to BASIC at power-on? BASIC will automatically switch out the PROMs and you'll have the full 64k memory available. Of course if you're teaching the class about bootloaders as well as BASIC, then this would be no good for you.

Tom L
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Re: Sharing peripherals

Postby mail@gabrielegan.com » June 5th, 2015, 4:38 am

Great idea, Tom, thanks. Will try that if/when the students get bored of toggling in the bootstrap code and using it to load the DBL from paper-tape. They still look at this happening like they're watching someone doing a magic trick. Noughts and ones coming out of holes in paper makes the experience of sound coming out of a spiral scratch in vinyl seem tame, and they certainly think the latter is "way cool". (British students may be some distance behind US students in their vocabulary of admiration.)

Thanks, Mike, for the pointers to specific further reading.

Regards

Gabriel
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