Great Hierophant
Veteran Member
As we know, Tandy 1000s came with one of two types of printer port :
The first was a 34-pin Card Edge port found on the 1000/A/HD/EX/SX/HX/TX/SL/TL/SL2/TL2. Their pinout is not widely known, but it is available from the various tech references.
The second was the standard female DB-25 port found on the IBM PC and compatibles. Their pinout is universally known.
Tandy's printers had 36-pin Centronics ports that adhered to a pre-IBM pinout. Not all Tandy printers use all the signals, and there is no pin on the Tandy Centronics for Auto Feed, Select or Select In.
When IBM sold its printers for the PC, it used the same connector but tweaked the pinout a bit. It does not provide +5v on pin 18 or a negative Busy signal. An excellent page showing the differences between the two is here :
http://support.radioshack.com/support_accessories/doc4/4631.htm
Tandy's Card Edge ports match the Tandy Centronics port pinout exactly, with pins 18 and 36 unconnected.
Tandy's DB-25 ports will work with an IBM port pinout with a standard printer cable.
But suppose you want to use a card edge port with an IBM-style printer? The adapter that needs to be made is not that complex, but there are a few issues that need to be addressed :
From Card Edge to IBM Centronics
Pin 25, *Busy not connected (1000/A/HD only)/Select In jumper enabled on other 1000s to Pin 36
Pin 32, *Auto Feed to Pin 14 (1000/A/HD only)
Pin 28, *Error to Pin 32
Pin 30, *Init to Pin 31
Pin 34, +5v to Pin 34 (NC) (EX and later 1000s only)
Essentially, if you have an EX or later model, you can make a straight Centronics adapter with just a twist to pins 31-32 on a ribbon cable and routing the Select In pin, if necessary. The original 1000 models require the Auto Feed pin to be routed if needed. Do not worry about pin 34 because the IBM Centronics port leaves it unconnected. The EX and the later Tandys have a jumper or a menu item to select whether Select In is sent to the card edge. Not all IBM Centronics printers use the signal. Select is not provided on the card edge, but in the pre TL/SL models is available from the printer port array chip. The PSSJ chip found in the TL/SL does not put out a Select signal.
If you want to use a Tandy 1000 with a DB-25 port with a Tandy Centronics printer, you will need to do the following:
Pin 13, Select not connected
Pin 14, *Auto Feed not connected
Pin 16, *Init to Pin 33
Pin 17, *Select In not connected
Pin 18, Not Connected (+5v on Centronics)
In short, simply cut pins 13, 18 and reroute pin 16.
Finally, I would hypothesize that the reason why nybble transfers from the unidirectional parallel port do not work in the 1000 series with card edges is that the Select signal is not provided at the card edge. The nybble data is formed from four signals : Error, Busy, Paper Out and Select. Since Select is not on the card edge, you are missing a bit. If you provided the signal, it may work. I wonder if anyone has ever tried this.
The first was a 34-pin Card Edge port found on the 1000/A/HD/EX/SX/HX/TX/SL/TL/SL2/TL2. Their pinout is not widely known, but it is available from the various tech references.
The second was the standard female DB-25 port found on the IBM PC and compatibles. Their pinout is universally known.
Tandy's printers had 36-pin Centronics ports that adhered to a pre-IBM pinout. Not all Tandy printers use all the signals, and there is no pin on the Tandy Centronics for Auto Feed, Select or Select In.
When IBM sold its printers for the PC, it used the same connector but tweaked the pinout a bit. It does not provide +5v on pin 18 or a negative Busy signal. An excellent page showing the differences between the two is here :
http://support.radioshack.com/support_accessories/doc4/4631.htm
Tandy's Card Edge ports match the Tandy Centronics port pinout exactly, with pins 18 and 36 unconnected.
Tandy's DB-25 ports will work with an IBM port pinout with a standard printer cable.
But suppose you want to use a card edge port with an IBM-style printer? The adapter that needs to be made is not that complex, but there are a few issues that need to be addressed :
From Card Edge to IBM Centronics
Pin 25, *Busy not connected (1000/A/HD only)/Select In jumper enabled on other 1000s to Pin 36
Pin 32, *Auto Feed to Pin 14 (1000/A/HD only)
Pin 28, *Error to Pin 32
Pin 30, *Init to Pin 31
Pin 34, +5v to Pin 34 (NC) (EX and later 1000s only)
Essentially, if you have an EX or later model, you can make a straight Centronics adapter with just a twist to pins 31-32 on a ribbon cable and routing the Select In pin, if necessary. The original 1000 models require the Auto Feed pin to be routed if needed. Do not worry about pin 34 because the IBM Centronics port leaves it unconnected. The EX and the later Tandys have a jumper or a menu item to select whether Select In is sent to the card edge. Not all IBM Centronics printers use the signal. Select is not provided on the card edge, but in the pre TL/SL models is available from the printer port array chip. The PSSJ chip found in the TL/SL does not put out a Select signal.
If you want to use a Tandy 1000 with a DB-25 port with a Tandy Centronics printer, you will need to do the following:
Pin 13, Select not connected
Pin 14, *Auto Feed not connected
Pin 16, *Init to Pin 33
Pin 17, *Select In not connected
Pin 18, Not Connected (+5v on Centronics)
In short, simply cut pins 13, 18 and reroute pin 16.
Finally, I would hypothesize that the reason why nybble transfers from the unidirectional parallel port do not work in the 1000 series with card edges is that the Select signal is not provided at the card edge. The nybble data is formed from four signals : Error, Busy, Paper Out and Select. Since Select is not on the card edge, you are missing a bit. If you provided the signal, it may work. I wonder if anyone has ever tried this.
