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Subject: 04 Printer Interfaces
Subject: 04.01 Centronics (parallel) Interface
Subject: 04.02 RS232 (serial) Interface
Subject: 04.02.01 Connecting an Apple LaserWriter to a PC
Subject: 04.03 RS422 (serial) Interface
Subject: 04.04 HP-IB (parallel) Interface
Subject: 04.05 HP-IL (serial) Interface
Subject: 04.06 SCSI Interface
Subject: 04.07 Infrared Interface
Subject: 04.08 Network Interfaces
Subject: 04.09 Interface Converters

Subject: 04 Printer Interfaces

Subject: 04.01 Centronics (parallel) Interface

The Centronics interface transfers data in a parallel fashion to a printer. As opposed to a serial cable that only sends data down one wire, the parallel protocol sends data down 8 wires at once, thus being generally much faster. The standard interface is only able to transfer data in one direction the "communication" in the reverse direction is done by status lines, e.g. paper error or busy), but there is a newer specification called Bi-Tronics (from HP) with bidirectional communication and higher transfer rates.

This interface is typically used for connecting a printer to a PC (with any OS) or a workstation, since the cable is very limited in length.

If you want the gory details, you can order the spec from the IEEE. Call 1-800-701-4333 and ask for document # SH17335. $56.50 plus $6.00 UPS ground shipping (From an article posted by customer@quake.net, dated 18 Apr 1995).

Basically, IEEE-1284 specifies the timing and protocol for several modes, including compatibility ("Centronics"), nibble (the reverse-channel part of HP's Bi-Tronics), and ECP (a fast, parallel bi-directional interlocked handshake). ECP should be able to transfer data in either direction at speeds approaching 2 MB/sec.

IEEE-1284 also specifies the electrical interface and cable length properties, and allows for cable lengths up to 10 meters. With pre-1284 specs (e.g. by HP) the maximum length was 3 meters.

The parallel connection may well work with longer cables (I have heard of 15 to 20 meters), but it very much depends on the setup (electrical wires near the cable may cause noise on the cable, power of the +5 volt supply, sensitivity of the printer's input decoder and so on). If you need a longer cable, you should consider using either a serial connection (RS232 or RS422) or a parallel repeater or extender kit.

In either case the maximum length is limited by IEEE-1284 since the maximum propagation delay of the cable must be less that 58 ns. When calculating with the speed of light this would result in a theoretical maximum length of about 17 meters.

Parallel interfaces run anywhere from less than 25 kbps to greater than one megabyte/sec, the high end is seen in newer PC's with ports supporting ECP/EPP/IEEE1284.

Subject: 04.02 RS232 (serial) Interface

The RS232 "standard" is one of those wonderful computer things everyone talks about and uses, but hardly anyone understands. This is a new attempt at explaining it, based upon the previous text, but with some more data about pinouts and other information.

The RS232 serial interface transfers asynchronous data bit by bit, one at a time. The bytes are separated from each other by one start bit and one or two stop bits. Besides the data lines this interface defines a couple of control lines between the two devices (see diagram below). This interface is able to transfer data simultaneously in both directions, allowing for printer status or error messages to be easily passed back to the computer.

The RS232 standard defines two types of equipment, DTE (computer, terminal, most printers) and DCE (some printers, modems). When connecting two DTE's you would need a null-modem cable, that is a cable which swaps TxD and RxD around and connects some control lines on both sides. The type of cable needed, especially which pins must be connected with each other, depends on the devices you want to connect. Note that some devices don't support all control lines.

There exist different connectors for the RS232 serial line. The standard type is called DB-25 (two lines of pins, one with 12 and the other one with 13 pins). There is another type called DB-9 (e.g. the HP LaserJet 4P is equipped with this type), this connector doesn't support all control lines, but it may well be connected with a DB-25 connector on the computer side.

And here are the pinouts for both DB-25 and DB-9 connectors (pins not mentioned are unused). The first number given is for the DB-25 type, the second number in parentheses is for the DB-9 type if supported. The text describes the common name for the pin and the signal direction from the device in question.

  1 (-)  Frame Ground (DB-9 type: use the connectors box instead)
  2 (2)  TxD: Transmitted Data (out)
  3 (3)  RxD: Received Data (in)
  4 (1)  RTS: Request To Send (out)
  5 (-)  CTS: Clear To Send (in)
  6 (4)  DSR: Data Set Ready (in)
  7 (5)  Signal Ground
  8 (-)  DCD: Data Carrier Detected (in)
 19 (-)  Reverse Channel (out)
 20 (8)  DTR: Data Terminal Ready (out)

Finally the pinout for a null-modem cable (with TxD and RxD swapped). In this diagram only the DB-25 pins are mentioned (use the pinout above for a DB-9 cable).

   1 -- Frame Ground -- 1
   2 --  TxD -> RxD  -> 3
   3 <-  RxD <- TxD  -- 2
   4 -+     connect  +- 4
   5 <+    RTS & CTS +> 5
   7 -- Signal Grnd. -- 7
   6 <+     connect  +> 6
   8 <+    DSR & DCD +> 8
  20 -+     with DTR +- 20

Note: when using this cable type (all control lines disabled) the computer can't determine whether the printer is on or off. Well, at least by means of the resp. control line; it is possible to check the printer state by using ETX/ACK handshake protocol. On the other hand some systems have problems when you connect the control lines correctly and the printer is off at boot time. (The system thinks that there is no serial line. There are some UNIX systems that block the open() system call until the printer is switched on.

Typical data transfer rates (for printer connections) are 9600 baud, 19200 baud and 38400 baud (baud is bits per second). Theoretically this interface may transfer data with up to 115200 baud (this is the maximum speed newer UART's can do, but with many systems you can't select this speed in a standard way - it's often necessary to program the UART by yourself). Although I'm told that the Lexmark IBM 4039 and Lexmark Optra printers support this speed, I haven't seen it with my own eyes. :)

Subject: 04.02.01 Connecting an Apple LaserWriter to a PC

Thanks to Tibor Pelan, we now know that there's a web page with the necessary information for making yourself a cable to connect your Apple LaserWriter to a PC using the serial port.

http://www-cs.intel.com/enduser_reseller/netport_print_servers/6021.HTM#3

The document describes the cable pinout and switch settings on several printers for use with Intel's NetPort Express. I don't have a LaserWriter anymore, so I can't doublecheck this, but I think the cable they describe should work fine with for connecting directly to your PC.

Subject: 04.03 RS422 (serial) Interface

The RS422 serial interface transfers synchronous data in blocks of bytes each of which begins with a synchronize byte and ends with a final character. For synchronization a sync line is supplied. There are separate lines for data transfer in each direction with two lines for each direction with complementary polarity. This interface is typically used for AppleTalk connections.

Transfer rates are up to 230.4 Kbaud for a 300 meter (~328.08 yard) cable. The maximum length is up to 1200 meters.

Subject: 04.04 HP-IB (parallel) Interface

HP-IB ("Hewlett-Packard Interface Bus") alias GP-IB ("General Purpose IB") alias IEEE-4888. Old and largely outdated interface created by HP and used for printers, plotters, disks, tape drives and measurement devices of various kinds, mostly by HP computers (HP3000 and HP9000 series, though not standard in current models), available also for PCs. Today used mainly for measurement devices, but HP-IB printers may still turn up in surplus sales. Using one in a PC requires either an HP-IB card or a parallel/HP-IB converter, both of which are hard to find and may cost more than the peripheral device. The connector resembles Centronics but locks with screws rather than clips.

Subject: 04.05 HP-IL (serial) Interface

HP-IL ("Hewlett-Packard Interface Loop"), was created by HP for their handheld and portable computers and calculators. Abandoned by HP a few years back, but HP-IL printers, notably the HP2225B ThinkJet, still turn up occasionally. It can be recognized by two small, rounded, trapezoidal-shaped connectors with two pins each. An HP-IL card for ISA-based PCs exists but is very hard to find these days, so if you find such a printer it's probably best to sell it to someone with an old HP-IL-based HP computer (HP41, HP71B, HP75 and HP-110 at least).

Subject: 04.06 SCSI Interface

SCSI (Small Computer Systems Interface) is best known as a mass storage interface, but SCSI-based printers also exist, including at least one Apple LaserWriter model and some really big and fast mainframe-class printers that can handle 100-200 ppm or more (at least Siemens and HP have such).

Subject: 04.07 Infrared Interface

Infrared interfaces have been used mainly in printers for HP calculators, but recently such have been announced for laptops as well, including an infrared-Centronics converter, so it may become more important in the future.

An organization called the Infrared Data Association (IrDA) has produced specification that are being adopted as industry standards. Several laptop and PDA vendors have come out with models supporting the IrDA specs, and many more manufacturers appear to be waiting in the wings. The IrDA spec provides for fully bi-directional communications at distances of 1 meter, at speeds up to 115,200 bits per second. An enhanced specification with much higher speeds is currently under proposal.

Recently (at the 1995 CeBIT trade show in Hannover, Germany) HP presented the new HP LaserJet 5P and 5MP models with an IrDA- compatible interface. Presumably other printer companies will follow soon.

Subject: 04.08 Network Interfaces

Network interfaces directly built into a printer are the most efficient way to transfer print jobs from any host in a network (LAN) to the printer. If the network is set up accordingly you may send print jobs from any host to the printer (with serial or parallel connected printers you have to route your print job to the host the printer is connected to). Standard connectors are 10base2, 10baseT and 10base5 (also called AUI).

In addition to the basic Ethernet connections, there are also 4Mb and 16Mb Token Ring (both STP and UTP) as well as LocalTalk.

Your network interface must support a communication method that is compatible with your network operating system(s). Examples include TCP/IP, AppleTalk, SPX/IPX, and DLC (a lower level interface frequently used in IBM LAN Server/MS LAN Manager/NT AS environments).

The maximum transfer speed depends on the network type (4, 10, 16 Mbit or 100 Mbit or more) and on the processing speed of the printer. With network interfaces you may well reach the maximum possible printer throughput, this is normally not the case with serial interfaces and maybe even not with parallel interfaces.

Subject: 04.09 Interface Converters

For connecting devices with different interfaces you may use a converter that maps one interface to another. It is possible to convert a serial RS232 interface into a Centronics parallel interface, or to convert a serial RS422 interface into a serial RS232 interface. Commercial solutions for these converters are available.

It is also possible to connect a printer without network interface to a network by means of a printer server. The best choice is a converter from network interface to a parallel interface (at least if you don't need bidirectional communication) since then you have the maximum possible throughput. With a converter to a serial interface you on the one hand lose transfer performance, but on the other hand you may get data back from the printer. In both cases you have the advantage that you can place the printer anywhere in your local network (typically near you), with a serial or parallel interface it's normally necessary to place the printer nearby the host it is connected to.

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Last modified: 04 Jan 1997 - bcm