All Parallel Ports Are NOT Created Equal

Developing a distance extender for PC parallel ports sounded like an easy process. Patton Electronics had already manufactured RS-232 to "just-about-anything" converters, not to mention a host of RS-232 type short range modems. After all, parallel ports are standard, right?

Wrong!

Just as there are hundreds of variations of "RS-232", parallel ports are also diverse. In fact, there is no written standard interface. The Centronics interface showed up as a de-facto standard in the mid- to late-70s and has been used ever since on printers and personal computers. But even these are not all the same. Patton's challenge was to develop a universal product that worked with the lowest common denominator parallel interface.

These ports use low TTL signals. Zero volts represents a low, 5 volts represents a logic high. While PC ports provide most of the power on the data leads (pins 2 - 9), the printer doesn't provide much power at all.

BUSY (pin 11) and ACKnowledge (pin 10) are used for data flow control on printer interfaces. The printer raises the BUSY signal (positive voltage)when it is processing data and cannot accept another character from the PC. After the printer receives and processes the character, it lowers ACK (zero voltage) and lowers BUSY to tell the PC to send another character. Some of the distance extenders on the market only monitor the ACK signal on theprinter end, some monitor BUSY. This may be a problem during print sharing. If the print sharing device monitors its BUSY lead and the distance extender generates the ACK pulse, flow control problems would result. Patton needed to build a unit that was universal in terms of flow control.

Why is the Model 1225 Different?
The Model 1225T (transmitter) operates in conjunction with the 1225R (receiver) to extend the distance of PC parallel ports to 2000 feet. The 1225T accepts the parallel data from the PC parallel port. This data is pulse width modulated and is transmitted serially to a 1225R over a single twisted pair wire. The 1225 converts the serial data back to the parallel format and sends it to the printer.

DC Power Transfer

The Model 1225T and 1225R derive power from the data and control signals. In addition, the primary windings of the 1225T are connected to the power supply. This allows the 1225T to transmit power down the 2 wire cable to the 1225R receiver.

BUSY or ACK Flow Control
The 1225T and the 1225R feature ACK or BUSY flow control using a conveniently located switch.

Using the ACK option: After the printer receives a negative-going strobe pulse from the computer, the printer responds by sending its ACK signal low.This pulse is sent from the 1225R over the twisted pair line to the 1225T. When the ACK signal is received, the 1225T lowers BUSY and send the ACKto the computer. When it sees ACK, the computer may send another character.

Using the BUSY option: When the printer raises BUSY, the 1225R unit sends short pulses to the 1225T. As long as the printer stays BUSY, ACK stays low to the PC. When BUSY drops, a longer ACK pulse is generated from the 1225T and the computer may send another byte of data.

A wide variety of parallel distance extenders are currently on the market. Some units work better than others. The reality is that until a parallel interface standard is developed and widely utilized, most Parallel Extenders will have problems operating in certain applications. Patton has taken a lowest common denominator approach to the design and therefore, should be the product of choice by most consumers.