Great Hierophant
Veteran Member
Early Tandy 1000s have a 34-Pin Card Edge port to connect to a printer. This port was taken from earlier Tandy devices from the TRS-80 line and was designed to be compatible with the pre-IBM Centronics standard. IBM added and changed a few status and control signals and used a DB-25 connector. Early Tandys 1000s sort of updated the port to include IBM's new signals, but not completely.
The early Tandys, 1000/A/HD/EX/SX/HX/TX, generate the printer port signals through a custom chip called the Parallel Port Gate Array. This chip provides 8 data bits, the status bits (Error, Select, Paper End, Acknowledge, and Busy) and the control bits (strobe, auto feed, initialize printer, select input, IRQ enable). All these signals are to be found on the IBM parallel port except IRQ enable. Tandy does not provide all the bits on its parallel port, even with an adapter. This is where you start to see incompatibilities with non-Tandy peripherals.
The later Tandys, the TL, TL/2, TL/3, RL, SL, SL/2, RLX, RSX, all include their parallel port logic inside the PSSJ chip. The earliest incarnation of this chip, codenamed Jacksboro, does not provide a Select line. Jacksboro can be found in the TL and SLs. The second revision of the PSSJ, codenamed Bonanza, is found in the TL/2, SL/2, RL, 2500XL and 2500XL/2 as well as some late manufactured TL and SLs. I am not sure whether the select line was restored. These chips may support a bidirectional parallel port, but it may be a non-PS/2 standard and the functionality was only offered on the RL. The third and fourth revisions of the PSSJ were codenamed Hensdale, found only in the TL/3, and Hensdale 2, found in the RLX, RSX, 2500RSX, 2500SX and Sensation!. This revision adds support for a PS/2 Bidirectional Parallel port.
The only Tandy 1000s to have a DB-25 port are the RL, TL/3, RLX and RSX. All older models have the Card-Edge port.
Another issue is that since the Tandy 1000s internalizes all the ordinary parallel port logic into one chip, there is no way to modify the interface to support a bidirectional mode, except for those machines that explicitly provide for it (RL, TL/3, RLX, RSX) Any devices requiring bidirectional mode, which includes many modern printers and many external drives, have no chance with working with any Tandy 1000 with a card edge.
So, whats missing? Well, Tandy never seemed to like the Select and Select Input signals, as it never connected both to the card edge connector on the same machine. Despite its name, Select Input is sent from the computer to the printer to select the printer. The printer sends the Select signal to indicate that it has been selected. It probably assumed that those signals were generally regarded as superfluous hand-shaking signals. The Acknowledge, Initialize Printer, and Busy signals can usually serve to communicate with a printer. Software decides what to do with all these signals anyway. The old Centronics standard had signals called Busy Negative and Initialize Negative (in addition to Positive versions), which seem to correspond to the function of Select and Select Input, respectively.
Among the parallel port devices which tend to have issues with the Tandys are the Iomega 100MB Zip Drives, the Xircom PE3 Ethernet Adapters and the Disney Sound Source. The Iomega and Xircom adapters can send data to a standard unidirectional parallel port at no more than five bits at a time, using the Status Bits. However, the Select signal is omitted from the card edge connector on every Tandy 1000 after the original, including the 1000A & HD. Thus you are missing a bit and reads from these devices must fail. If you are sufficiently brave, you can solder a wire from pin 16 of the Parallel Port Array chip of the pre-PSSJ models to an unconnected pin on the card edge. Unless Tandy decided to reconnect the pin on its Bonanza or Hensdale PSSJs, you are out of luck with nibble mode. However, since the Hensdale PSSJs support the PS/2 Bidirectional Mode anyway, you are better off.
The Select Input signal can be sent to the card edge on the 1000A, 1000HD, 1000EX, 1000SX, 1000HX, 1000TX, 1000TL, 1000SL, 1000SL/2 via jumper. On the 1000RL, there is an option for it, but the user must install jumper pins himself. The original 1000 requires a wire from pin 26 of the parallel port array. There are no jumpers for it for the TL/2 or other 1000s or the 2500XL and 2500XL-2.
The Disney Sound Source claims that it does not work with the RL, TL/3 or 2500XL. Disney uses the 8-data pins and the Acknowledge, Initialize Printer and Select Input lines. However on Tandy with a card edge it uses a special adapter, and because the Select Input signal cannot be guaranteed to be available, the Auto Feed signal is used instead. When Disney released games for the Sound Source, it probably did not account for the Tandys initially, because different values must be sent to the parallel port to get the Sound Source to work correctly on Tandys with less than full compatibility. It seems that the Hensdale-2 chip in the RLX and other machines put the Select Input back on the connector, as games do not include later Tandys on the list of computers which do not work with the Sound Source. I assume that the line is not connected on the TL/2, 2500XL or 2500XL-2. If the user added the jumper to the RL, then the Sound Source should become fully compatible.
The early Tandys, 1000/A/HD/EX/SX/HX/TX, generate the printer port signals through a custom chip called the Parallel Port Gate Array. This chip provides 8 data bits, the status bits (Error, Select, Paper End, Acknowledge, and Busy) and the control bits (strobe, auto feed, initialize printer, select input, IRQ enable). All these signals are to be found on the IBM parallel port except IRQ enable. Tandy does not provide all the bits on its parallel port, even with an adapter. This is where you start to see incompatibilities with non-Tandy peripherals.
The later Tandys, the TL, TL/2, TL/3, RL, SL, SL/2, RLX, RSX, all include their parallel port logic inside the PSSJ chip. The earliest incarnation of this chip, codenamed Jacksboro, does not provide a Select line. Jacksboro can be found in the TL and SLs. The second revision of the PSSJ, codenamed Bonanza, is found in the TL/2, SL/2, RL, 2500XL and 2500XL/2 as well as some late manufactured TL and SLs. I am not sure whether the select line was restored. These chips may support a bidirectional parallel port, but it may be a non-PS/2 standard and the functionality was only offered on the RL. The third and fourth revisions of the PSSJ were codenamed Hensdale, found only in the TL/3, and Hensdale 2, found in the RLX, RSX, 2500RSX, 2500SX and Sensation!. This revision adds support for a PS/2 Bidirectional Parallel port.
The only Tandy 1000s to have a DB-25 port are the RL, TL/3, RLX and RSX. All older models have the Card-Edge port.
Another issue is that since the Tandy 1000s internalizes all the ordinary parallel port logic into one chip, there is no way to modify the interface to support a bidirectional mode, except for those machines that explicitly provide for it (RL, TL/3, RLX, RSX) Any devices requiring bidirectional mode, which includes many modern printers and many external drives, have no chance with working with any Tandy 1000 with a card edge.
So, whats missing? Well, Tandy never seemed to like the Select and Select Input signals, as it never connected both to the card edge connector on the same machine. Despite its name, Select Input is sent from the computer to the printer to select the printer. The printer sends the Select signal to indicate that it has been selected. It probably assumed that those signals were generally regarded as superfluous hand-shaking signals. The Acknowledge, Initialize Printer, and Busy signals can usually serve to communicate with a printer. Software decides what to do with all these signals anyway. The old Centronics standard had signals called Busy Negative and Initialize Negative (in addition to Positive versions), which seem to correspond to the function of Select and Select Input, respectively.
Among the parallel port devices which tend to have issues with the Tandys are the Iomega 100MB Zip Drives, the Xircom PE3 Ethernet Adapters and the Disney Sound Source. The Iomega and Xircom adapters can send data to a standard unidirectional parallel port at no more than five bits at a time, using the Status Bits. However, the Select signal is omitted from the card edge connector on every Tandy 1000 after the original, including the 1000A & HD. Thus you are missing a bit and reads from these devices must fail. If you are sufficiently brave, you can solder a wire from pin 16 of the Parallel Port Array chip of the pre-PSSJ models to an unconnected pin on the card edge. Unless Tandy decided to reconnect the pin on its Bonanza or Hensdale PSSJs, you are out of luck with nibble mode. However, since the Hensdale PSSJs support the PS/2 Bidirectional Mode anyway, you are better off.
The Select Input signal can be sent to the card edge on the 1000A, 1000HD, 1000EX, 1000SX, 1000HX, 1000TX, 1000TL, 1000SL, 1000SL/2 via jumper. On the 1000RL, there is an option for it, but the user must install jumper pins himself. The original 1000 requires a wire from pin 26 of the parallel port array. There are no jumpers for it for the TL/2 or other 1000s or the 2500XL and 2500XL-2.
The Disney Sound Source claims that it does not work with the RL, TL/3 or 2500XL. Disney uses the 8-data pins and the Acknowledge, Initialize Printer and Select Input lines. However on Tandy with a card edge it uses a special adapter, and because the Select Input signal cannot be guaranteed to be available, the Auto Feed signal is used instead. When Disney released games for the Sound Source, it probably did not account for the Tandys initially, because different values must be sent to the parallel port to get the Sound Source to work correctly on Tandys with less than full compatibility. It seems that the Hensdale-2 chip in the RLX and other machines put the Select Input back on the connector, as games do not include later Tandys on the list of computers which do not work with the Sound Source. I assume that the line is not connected on the TL/2, 2500XL or 2500XL-2. If the user added the jumper to the RL, then the Sound Source should become fully compatible.