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The following description of the operation of the interface is based on information in the Star SG10 printer manual.

Pin 1 (STROBE-). Carries the STROBE- pulse signal from the computer to the printer, this is normally held high by the computer. When the computer has data ready for the printer it sets this signal low for at least 0.5uS. When the printer sees this pulse on the strobe pin, it reads the data that the computer supplies on pins 2 through 9 (DATA1 to DATA8). The computer must maintain this data for a period beginning at least 0.5uS before the strobe pulse starts and finishing at least 0.5uS after the strobe pulse ends. When the printer has successfully received the byte of data from the computer it sets pin 10 (ACK-) low for about 9uS.

Pin 2 to Pin 9 (DATA). See above.

Pin 10 (ACK-). See above.

Pin 11 (BUSY). Reports when the printer is unable to receive data. This signal will be high during data transfer, while the printer is actually printing a line of characters rather than receiving, when the printer is off-line and when an error condition exists.

Pin 12 (PAPER OUT). Reports when the printer is out of paper.

Pin 13 (SELECT). Indicates that the printer is on-line.

Pin 14 (AUTO FEED-). This function is not supported on most printers. When this line is low, the printer does an extra line feed at the end of the data. Since the PC always holds this line high, its function is ignored on the Printer Sharer and the line is held high via a resistor.

Pins 16 and 17. Ground.

Pin 18 (Power Supply Output). Normally connected to the +5V supply inside the printer - the Epson Stylus 400 is the only exception found, the pin being unconnected. The line is normally used to power the printer sharer, providing it can deliver about 100mA.

Pins 19 to 30. Ground.

Pin 31 (RESET-). Can be used to reset the printer. The "Energy Star compliant" printers monitor this pin for the automatic shut-off function - the printer goes into standby mode if the voltage on this pin falls below about 1V for longer than about half a second.

Pin 32 (ERROR-). Used to report an error condition in the printer.

Pin 33. Ground.

Pin 35. One of life's mysteries! Most printer manuals describe it as unused, but one described it as an output and said it was pulled high via a 1K resistor. It is not connected through to the PC so it has been ignored!

Pin 36 (SELECT IN-). Unused on most printers. Only one printer has been found that uses it - the InfoRunner Riteman II, which does not print the data it receives when the line is high! This seemed utterly pointless and so this pin on the Printer Sharer has been held low.

In some cases, the various output lines from the printer are open-collector, so pull-up resistors are included within the printer sharer (R11 to R13, R29 to R32).