Unidirectional Printer Ports
The first parallel printer port for the IBM PC had eight data lines, five status lines and four control lines. The data lines allowed the computer to send eight bits of data to the printer. The four control bits would send control information to the printer, which could send five bits back to the PC on the status bits. Thus, the parallel port has three registers, and in a system without a monochrome display adapter parallel port, their addresses are these :
0378 - Data Port
0379 - Status Port
037A - Control Port
The status port is read only, and the data and control ports are read and write capable. If the data or control port is read, the computer will read the bits last sent to the port. IBM warned against a printer or other external device trying to use the parallel port to write data on the lines meant for the data or control ports. Thus these ports are designated as output only ports. Officially, a printer communicated to the computer through the use of the status port. This port is designated as an input only port.
The bit assignments for the status and control ports become important to the discussion below :
| Line Name | IBM Pin | Active Low | Direction | Function | Bit |
| Strobe | 1 | Yes | Out | Control Port 37A | C0 |
| Data 0 | 2 | No | Out | Data Port 378 | D0 |
| Data 1 | 3 | No | Out | Data Port 378 | D1 |
| Data 2 | 4 | No | Out | Data Port 378 | D2 |
| Data 3 | 5 | No | Out | Data Port 378 | D3 |
| Data 4 | 6 | No | Out | Data Port 378 | D4 |
| Data 5 | 7 | No | Out | Data Port 378 | D5 |
| Data 6 | 8 | No | Out | Data Port 378 | D6 |
| Data 7 | 9 | No | Out | Data Port 378 | D7 |
| Ack | 10 | Yes | In | Status Port 379 | S6 |
| Busy | 11 | No | In | Status Port 379 | S7 |
| Paper-Out | 12 | No | In | Status Port 379 | S5 |
| Select (Input) | 13 | No | In | Status Port 379 | S4 |
| Auto Linefeed XT | 14 | Yes | Out | Control Port 37A | C1 |
| Error | 15 | Yes | In | Status Port 379 | S3 |
| Init | 16 | Yes | Out | Control Port 37A | C2 |
| Select-In (Output) | 17 | Yes | Out | Control Port 37A | C3 |
| Ground | 18-25 |
Strobe - The strobe signal is sent to the printer to alert it that there is valid data byte on the data output lines. This signal is active low and held for at least 0.5uS. AKA STB,
Ack - The acknowledge signal is sent from the printer to tell the computer that the printer has received the data byte and ready to accept more data. This line is tied to IRQ7 through the IRQ enable bit, Control Port C4. This signal is typically held low for 5uS. AKA ACKNLG, PACK
Busy - The busy signal is sent from the printer to tell the computer, when high, that it cannot process data at the present time. This is typically when the printer is busy processing data, printing data, when it is offline or has reported an error.
Paper Out - The out of paper signal is sent from the printer and, when high, tells the computer that the printer is out of paper. AKA End of Form, P. End, PE, PAEM, Out of Paper
Select Printer (Status Port) - The printer selected signal is sent from the printer to indicate to the PC that, when high, the printer considers itself selected and on-line. AKA Printer Selected, SLCT, SEL
Auto Linefeed XT - The auto linefeed signal is sent to the printer to tell it, when low, to advance the printer a line after a carriage return. AKA AUTO FEED XT, AF
Error - The error signal is sent from the printer to the computer to alert the computer of a fault at the printer's end. This is typically when the printer is offline, out of paper or has some error condition. AKA FAULT
Init - The initialize printer signal is sent to the printer to tell it to reset itself and clear its printer buffer. This signal must be held low for 50uS for it to be effective. AKA PRIME
Select-Input (Control Port) - The printer select signal is sent to the printer to tell it that, when low, it has been selected and should be ready to receive data. AKA SLCT IN, Select-In
One thing to note about these bits is that of the Control Port signals, the Strobe, Auto Linefeed and Select-In (Output) are all inverted as they are output from their register to the connector. Since these are active Low, you must set these bits High (1) to activate these signals. Also, the Busy signal, as it comes from the printer, is also inverted at the Status Port. Since Busy is active High, a Low (0) bit means the Printer is Busy.
Not all bits are used by all printers. Most printers have a dipswitch to set the printer to the Auto Linefeed mode, since this usually desired. It is hard to imagine today that a printer or typewriter would not automatically advance to the next line after pressing the Carriage Return/Enter key.
IBM's Graphics Printer, essentially the standard of the day, had a dipswitch to always set Select Printer (which from the printer's perspective it is an output line) to the active state.
C5 is used as an Interrupt Enable line. It is active High, so when a (1) is written to the bit, IRQ7 will be sent when the Ack signal becomes active. The intended use for this signal is that the printer program, when the IRQ was sent, would know without having to waste time polling the Status Port that it could send the next byte to the printer.
Nibble Mode
Before the introduction of the PS/2 Bidrectional Mode, the Nibble mode was the only compatible way to read data from an external device connected to the parallel port. The external device would drive its data onto the status port. The high four bits of the port would be the easiest to use, which in this case would be the Busy, Ack, Paper Out and Select bits. This would form a 4-bit nibble, and two nibbles would form a byte. As the Busy signal is inverted, code would need to consider that in obtaining the right data nibble.
The Status Port is a 5-bit port, and if read eight times could be reformed into five bytes by including the Error bit in the read.
PS/2 Bidirectional Mode
In 1987, IBM released the PS/2 and it boasted of a bidirectional parallel port mode that would allow an external device attached to the parallel port to send eight bits of data to the computer via the data port. TTL-based parallel ports for the IBM PC family, including IBM's, could easily be modified to do this. In this case, Control Port C5, when set (1), would change the data output port to a data input port. The default behavior was that C5 was not set (0). When in data input mode, a read on the data port will give the value sent by the external device. A write to the port would do nothing.
IBM's PS/2 and PS/1 family had more advanced bidirectional capabilities involving new registers outside the parallel port I/O space, but the basic functionality described in the previous paragraph is all that could be assumed on a regular PC compatible.
Tandy 1000
While there are the later EPP and ECP parallel ports, this post is primarily based on what the built-in ports of the Tandy 1000 are capable of, and none of them are capable of such advanced functionality.
Tandy 1000 printer ports are based off the pre-IBM Centronics standard and have some differences. Tandy marketed many, many printers, and they were designed to work with all its computer lines, including the TRS-80, the Color Computer and the Tandy 1000, 2000, 1200, 3000 & 4000. Early printers did not use all the control and status lines of the IBM parallel printer port. The two handshaking signals, strobe and ack, are the most important for any printer.
The Select signals are not used on Tandy printers. Auto Linefeed is not used by some Tandy printers, the Init signal is not used by all Tandy printers and they provide an active low as well as an active high Busy signal on two separate pins. Some Tandy printers also provide +5v on their Centronics connector at pin 18, you will want disconnect this pin at the DB-25 end of the cable if you use one of these printers. Fortunately, pin 18 is unconnected on standard Centronics 36-pin male to DB-25 male printer cables. Tandy Centronics connectors put Init, when used, at Pin 33, whereas it is on Pin 31 on IBM Centronics connectors. This is the functional difference between Tandy Type A and B Card-edge to Centronics cables and Type C and D DB-25 to Centronics cables.
The Tandy 1000/A/HD have a 40-pin Parallel Port Array IC, it differs slightly from the later Array used on the SX, HX, EX and TX. The technical reference implies that it is capable of bidirectional operation. One difference is that the Auto Linefeed signal is on pin 32 of the 1000/A/HD card edge. On later machines this signal is found on pin 27. The Tandy 1000/A/HD connector leaves pin 34 unconnected, but later Tandys connect it to +5v.
The EX, HX, SX and TX all share the same Parallel Port Array IC and their ports are wired identically. While the IC has a pin (16) for the Select-Input (Control Port) line, it is not connected to any of the lines on the card edge. Additionally, all five Status Port lines are connected from the card edge to the IC, so theoretically the port is capable of nibble transfers. However, the Select Printer (Status Port) line usually needs to be connected via jumper to the Array IC because Tandy's printers do not use it).
Also, in theory, you could connect the Select-Input (Control Port) to the card edge and then to a parallel device by a custom adapter. However, you would need a pullup resistor and a capacitor as the other lines have.
The TL and SL use the Jacksboro version of the PSSJ chip to implement a parallel port. This chips does not have a pin for the Select-Input (Control Port) line at all. Ditto for the TL/2, SL/2 and RL, even though they use the more advanced Bonanza PSSJ. The number of pins on either version of the chip are the same. Like the earlier devices, the Select Printer (Status Port) needs to be enabled via jumper. On the TL/2 and RL, there are no jumper standoffs, so the jumper pins will need to be shorted.
The TL/3, RLX and RSX use the Hensdale version of the PSSJ, which has more pins and have a regular, bidirectional DB-25 parallel port. The Select-Input (Control Port) and Select Printer (Status Port) lines are present on the connector.
Contructing a Converter Cable
| Line Name | IBM Pin | Tandy Pin |
| Strobe | 1 | 1 |
| Data 0 | 2 | 3 |
| Data 1 | 3 | 5 |
| Data 2 | 4 | 7 |
| Data 3 | 5 | 9 |
| Data 4 | 6 | 11 |
| Data 5 | 7 | 13 |
| Data 6 | 8 | 15 |
| Data 7 | 9 | 17 |
| Ack | 10 | 19 |
| Busy | 11 | 21 |
| Paper-Out | 12 | 23 |
| Select Input | 13 | 25 |
| Linefeed | 14 | 32/27 |
| Error | 15 | 28 |
| Init | 16 | 30 |
| Select-In Output | 17 | |
| Ground | 18-25 | see below |
Fortunately, the 34-pin card edge connector used by the Tandy was also used by 5.25" floppy drives. The Tandy printer card edge connector is not keyed while the floppy connectors usually are, so you will need to get an unkeyed connector or cut the plastic inside. A short ribbon cable and a DB-25 female port connector is all that is required. As you can see, the cable is very straightforward until IBM pin 15. Never connect the card edge pin 34 to anything on a DB-25 connector. Pins 14, 26 and 32 are often grounded or unconnected, you can use one of the unconnected pins you try to connect Select In (Output). Pins 2, 4, 6, 8, 10, 12, 16, 18, 20, 22, 24, 31 and 33 are connected to ground, but one Tandy ground wire for each DB-25 ground pin should be fine.
Covox Speech Thing & Disney Sound Source
The Disney Sound Source is an 8-bit D/A converter that is housed in a parallel port dongle and connected to a speaker. The circuitry inside the dongle is powered by a 9V battery in the speaker, and a 4-pin wire with an RJ-11 jack connects the dongle to the speaker. It does not require a bidirectional parallel port, but it does have a problem with the Tandy card edge port.
The problem is that the Sound Source, in addition to the eight data lines, uses the Ack, Init and Select-Input (Control Port) lines to control the D/A converter. As the card edge does not include the Select-Input (Control Port) line, the adapter will not work correctly in a Tandy. Disney indicated that with a special adapter, the Auto Linefeed line could be used instead, but this would require a small modification to the code needed to run the Sound Source.
The Covox Speech Thing is a simpler device that does not use anything other than the data port, which is connected to a purely-passive resistor network to an audio jack. The Tandy PSSJ DAC can emulate a Covox device using a program called Setdac, assuming the Covox-supporting program obeys DOS LPT address assignments.
Xircom PE3 Series Parallel Port Ethernet Adapters
Iomega Parallel Port Zip-100 Drives
MediaVision Audioport
Will only work with the ports in the TL3, RLX and RSX. It is possible it may work in a TX or older if the Select-Input (Control Port) line is brought to the connector.
Digital Joystick Adapters
The parallel port can be used as an adapter for simple, Atari-style digital joysticks. This requires the game have specific support for this method. Dyna Blaster is an example of a game that came with such an adapter. In this case, the adapter had two DE-9 ports for Atari joysticks. As you may recall from a prior post, there are many, many devices that are compatible with the Atari port. For a game released in 1992 like Dyna Blaster, the user could use a spare Sega Master System or Mega Drive (Genesis) controller.
The maker of the adapter generally would not assume that the user would know the type of parallel port he had (unidirectional, bidirectional, EPP or ECP) or its setting. Using the basic unidirectional port, five inputs bits are available on the status port. Coincidentally, the basic Atari CX-40 joystick has four directional contacts and one button. Each one can use one status bit to signal whether the directional or button has been pressed. More buttons cannot be supported in this manner without activating some kind of multiplexer in the joystick.
Each button and joystick position hovers over an open switch. The switch is closed when the user presses the joystick in a direction or the button down. One contact of each switch is connected to a separate wire on the cable, and the other half of every switch is connected to a common wire. Using one of the four control or eight data bits for the common wire, twelve joysticks could be controlled in this way. The joystick essentially becomes a loop connector.
The Atari joystick has no chips inside it, nor does a Sega Master System or Atari 7800 controller, but the Genesis, NES and SNES controllers all have a logic chip or two that requires +5v. Fortunately, the Tandy card edge provides +5v on pin 34 (except for the 1000/A/HD) Using a dedicated line is much easier on the port than trying to combine the power with parallel port data pins as the NES and SNES adapters used by SNESkey do.
Parallel to SCSI Adapters
I take no credit for this discovery, that goes to my friend Cloudschatze, but there does exist a parallel port to Shuttle SCSI adapter that apparently works with the Tandy card-edge port to transfer data in both directions. Shuttle made an adapter with a driver that will work in a pure bidirectional mode and overlook the lack of a Select-Input (Control Port) line, unlike other adapters. Therefore, with the card edge to DB-25 converter, the Shuttle adapter and a SCSI storage device, it is quite possible to get the parallel port to transfer data to the Tandy, at least the *X or better models.
LapLink Cable
It is quite possible to get the MS-DOS program Interlnk to work with the Tandy 1000s and their parallel ports, see here for details :
http://nerdlypleasures.blogspot.com/2014/09/tandy-1000s-and-interlnk-and-laplink.html
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