50 pin D-SUB male connector at the computer.
50 pin D-SUB female connector at the cable.
The Dataproducts parallel interface is a de facto industry standard developed by Dataproducts Corporation for its line printers (Models 2230, 2420, 2440, 2470 and successors) from the early 1970s onwards. It uses an 8-bit unidirectional data bus with a Paper Instruction (PI) line and a high-true Data Strobe/Demand handshake, operating at TTL levels and supporting transfer rates up to 300,000 characters per second. The printer-side connector on the short-line (single-ended) variant is a 50-pin D-sub (DD-50); the host/controller side uses a 37-pin D-sub (DC-37). DEC adopted this interface for its LP-series line printers (LP25, LP26, LP27, LP29, LP32, LP37, LG01, LG02), connecting them via the BC27A cable to LP11 and LPV11 UNIBUS controllers.
| Pin | Return | Dir | Description | Active State |
|---|---|---|---|---|
| 22 | 6 | IN | Ready | High |
| 21 | 5 | IN | On Line | High |
| 23 | 7 | IN | Demand | High |
| 38 | 37 | OUT | Data Strobe | High |
| 19 | 3 | OUT | Data 1 | n/a |
| 20 | 4 | OUT | Data 2 | n/a |
| 1 | 2 | OUT | Data 3 | n/a |
| 41 | 40 | OUT | Data 4 | n/a |
| 34 | 18 | OUT | Data 5 | n/a |
| 43 | 42 | OUT | Data 6 | n/a |
| 36 | 35 | OUT | Data 7 | n/a |
| 28 | 44 | OUT | Data 8 | n/a |
| 29 | 13 | OUT | Parity | n/a |
| 50 | 32 | IN | Ident 0 | n/a |
| 49 | 16 | IN | Ident 1 | n/a |
| 46 | 45 | OUT | Interface Verify. Tied together. | Low? |
| 12 | 39 | OUT | +5 VDC (Test) | High |
| 27 | 11 | IN | Parity Error | High |
| 25 | 9 | IN | Bottom of Form | High |
| 24 | 8 | IN | Top of Form | High |
| 30 | 14 | OUT | Paper Instruction | High |
| 31 | 15 | OUT | Buffer Clear | High |
| 26 | 10 | IN | Paper Moving | High |
| 48 | 17 | IN | Paper Moving | High |
| 47 | 33 | IN | Not VFU | High |
Notes
- Direction is Host (Computer) relative Peripheral (Printer).
- Return is Ground signal for Unbalanced and the Negative Signal for Balanced.
- The short-line (single-ended) variant supports cable runs up to approximately 40 feet (12 m). A long-line (differential/balanced) variant uses the same 50-pin connector but treats the return lines as the negative differential signal rather than ground, extending the maximum cable run to 500 feet; an optional long-lines adapter can extend this further to 2,000 feet.
- Data Strobe polarity is configurable on some implementations: leading-edge, middle (default), or trailing-edge sensing.
- Interface Verify pins (pin 46 and associated returns) are tied together as a loopback check to confirm cable integrity.
- The +5 VDC (Test) pin (pin 12) provides a reference voltage used for interface verification, not as a power supply to the host.
- The Dataproducts interface is not pin-compatible with the Centronics parallel interface: Centronics uses an active-low Data Strobe and active-low ACKNLG on a 36-pin micro-ribbon connector, whereas Dataproducts uses a high-true Data Strobe and a Demand handshake on a 50-pin D-sub.
Signals
- Ready — high-true output from printer; asserted when AC and DC power is present, paper is loaded, and no check condition exists.
- On Line — high-true output from printer; asserted when Ready is true and the operator has activated the ONLINE switch.
- Demand — high-true output from printer (also called Data Request); synchronises host data transmission with printer timing, replacing the Centronics BUSY/ACKNLG mechanism.
- Data Strobe — high-true input to printer from host; clocks the data lines into the printer. Pulse timing (leading, middle, or trailing edge) is configurable.
- Data 1–Data 8 — eight parallel data lines from host to printer, TTL levels; carry the character byte to be printed.
- Parity — odd or even parity bit transmitted alongside the data byte; function varies by printer model.
- Ident 0, Ident 1 — printer-to-host identification lines; function not publicly documented for all models.
- Interface Verify — loopback pins tied together at the printer connector to allow the host to verify cable and connector integrity.
- +5 VDC (Test) — printer outputs +5 V reference on this pin for interface verification purposes.
- Parity Error — high-true output from printer; asserted when a parity mismatch is detected on received data.
- Bottom of Form — high-true output from printer; signals that the paper has reached the bottom-of-form position (also carries IBOF — Ignore Bottom of Form — on some models when configured).
- Top of Form — high-true output from printer; signals that the paper has reached the top-of-form position.
- Paper Instruction — high-true input to printer from host; carries the Dataproducts Vertical Format Unit (DVFU) signal with the same timing and polarity as the data lines.
- Buffer Clear — high-true input to printer from host; clears the printer’s data buffer.
- Paper Moving — high-true output from printer; asserted while the paper transport is in motion (two pins carry this signal).
- Not VFU — high-true output from printer; indicates the vertical format unit is not ready or not fitted.
References
- Printronix P6000L/P6200L Line Printers User’s Reference Manual — Chapter 7: Interfaces
- DEC Cables Reference Guide (BC27A pinout, LP25/LP26/LP27/LP32)
- LP11/LS11/LA11 Line Printer User’s Manual (EK-LP11S-OP-001)
- Dataproducts line printer technical manuals, Models 2230, 2420, 2440, 2470 (Computer History Museum, catalogue 102753073)