neff@hpvcfs1.HP.COM (Dave Neff) (06/08/89)
There have been recent discussions on this, and other forums regarding the pullup resistors and resultant currents when a DeskJet and DeskJet+ is used on the Atari ST. It seems the Atari is limitted to 1.6 ma of current sink, yet the DeskJet printers use 2.2K pullup resistors on the data lines (5v/2.2 gives 2.3 ma) and a 1.1K pullup resistor on the strobe line (5v/1.1 gives 4.5 ma). Some have argued the DeskJets are disobeying the Centronics specification and others are concerned about damage to the Atari ST. Let me respond to these concerns: According to "Interface Specifications", Centronics Data Computer Corporation, Copyright 1977, and I quote: Interface Circuit Specifications: Line Terminaion: The printer interface terminates input data lines DATA1-DATA8 with 1000 ohms to +5 Volts and control lines DATA STROBE (bar) and INPUT PRINT (bar) with 470 ohms to +5 volts. (end quote). Hence the DeskJet printers use larger values than the specification resulting in less current drain than even the specification would require. Of course the above document also has the line "Specifications Subject to Change without notice" :-). These specifications were of course for a particular printer board built by Centronics. The actual pullup resistors used vary radically from printer to printer. As for concern of damage to the sound chip in the ST, evidently the ST sound chip can only sink 1.6 ma of current. A small pullup resistor will not cause it to sink any more than 1.6 ma, rather it will limit the lower bounds on logic level 0. This should not damage the sound chip but marginal voltage levels could result in invalid data or confused handshakes. The logic level 0 must be below .8 V for proper functioning of the Centronics port with a DeskJet or DeskJet+. I have heard from a number of Atari ST users who report no damage when using the printer with the Atari. One user (bovet@hao.UCAR.EDU) reports his experience using a Daisy Wheel printer which had 180 ohm pullup resistors. The Daisy Wheel did not function but there was no damage to the sound chip. To insure proper functioning of Atari STs with DeskJet family printers it would probably be best to put a buffer circuit between the Atari and the DeskJet. Design of the circuit is left as an excercise to the reader :-). Dave Neff hplabs!hpvcfs1!neff