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From: bmw@aesat.UUCP (Bruce Walker)
Newsgroups: net.micro
Subject: Re: hardware question (schematic)
Message-ID: <207@aesat.UUCP>
Date: Thu, 23-Aug-84 09:16:07 EDT
Article-I.D.: aesat.207
Posted: Thu Aug 23 09:16:07 1984
Date-Received: Thu, 23-Aug-84 13:10:23 EDT
References: <502@bunker.UUCP>
Organization: AES Data Inc., Mississauga Ont., Canada
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<>

> I have an old TI thermal printer (PC100A) that works with their
> 58/59 calculators.  I thought it might be kind of neat to interface
> it to my computer.
> 
> problem : The five unidirectional logic lines in the interface
>           use a 0 volt to -10 volt logic.
> 
> question:  Is this TI logic some sort of standard?
>            Is there an easy way to interface it to TTL ?
> 
> Someone suggested a high speed op amp, but isn't this overkill?
> (the clock frequency is about 227khz ).
>                         
>                                Jeff Hagen
>                              ittvax!bunker!jrh


This logic is a P-channel MOS standard from old calculator days.  The trouble
with using your usual opamp in this application is the low slew rate.  Using a
high-speed opamp to correct this would be overkill.

Here's a fairly simple (and cheap!!) circuit which could work for your thermal
printer interface:


   TTL input ---> |
                  |          +12 V
                +-+-+          ^
                |   |          |
              ========= 14     |
   RS232      |  1/4  |--------+
line driver   | of a  |
(Gnd: pin 7)  | 1488  |--------+
               \_____/  1      |
                  O            |
                  |            V
                  |          -12 V
                  |
                  |      1K 5% 1/4W
                  +-----/\/\/\/----+-------->   Output: 0 to -10 V
                  |                |
   1N914 or     -----              \
   1N4148       \   /              /
general purpose  \ /       10K 5%  \
silicon         =====              /
switching diode   |                \
                  |                |
                  |                V
                  |              -12 V
                  |
                 === (common system ground)
                 ///

One channel is shown.  There are 4 drivers in one 1488 package, so you'll need
two packages.  Any old 1488 will do: TI (75188), Fairchild, Motorola etc.

The buffer IC was chosen because of its popularity, there are interface IC's
specifically for this kind of job but they are more expensive and/or scarce.

The circuit was taken from the "Applications" section of the MC1488 data sheet
in the Motorola 1983 "Linear & Interface IC's" data book.


Bruce Walker     {allegra,ihnp4,linus,decvax}!utzoo!aesat!bmw