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https://github.com/rhalkyard/plotadapter

Serial-GPIB converter for HP plotters
https://github.com/rhalkyard/plotadapter

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Serial-GPIB converter for HP plotters

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README 

        
# PlotAdapter: Serial-GPIB converter for HP Plotters



## Introduction



Older Hewlett-Packard pen plotters usually had an RS232 interface, but they were

also available with GPIB (a.k.a. HP-IB, a.k.a. IEEE-488), a popular interface

for laboratory equipment, also seen on HP's own 9800 series of computers.



While GPIB interfaces are still available, both as (expensive) commercial

products and as open-source hardware, they are not well-suited to the task of

controlling a plotter - most PC software expects to talk to a plotter over a

serial port, not the VISA APIs typically used by GPIB adapters. Additonally,

many of the serial-port-based GPIB adapters are more suited to

instrument-control applications - sending and receiving short commands and

responses - and as such lack the flow-control mechanisms needed for successful

plotter operation.



Furthermore, HP's RS232 plotters supported a wide range of serial handshaking

options, and implemented an additional set of escape sequences to configure

this. Many applications send these escape sequences before and during plotting,

and expect the plotter to respond accordingly - for example, HPGL plots

generated by Autocad contain escape sequences to put the plotter into

`XON`/`XOFF` handshaking mode.



PlotAdapter aims to emulate this extended command set, allowing a GPIB plotter

to be 'converted' to an RS232 interface and used by software that expects to

communicate with an RS232-connected plotter.



## Getting Started



### Hardware



You will need:



* An HPGL plotter with a GPIB interface (e.g. a 7470A with Option 002)



* An AVR-based Arduino with 5 volt IO and 14 available pins (I am using a Mega

  2560)



* A GPIB cable with some way of connecting it to the Arduino



* (optional) a USB-serial interface with support for serial handshaking (see

  "Flow Control Limitations" section below for details)



### Software



It is crucial to use some form of flow control when sending data to the plotter.

Many plotter operations are time-consuming, and without flow control, it is very

likely that a buffer overrun will occur. This is complicated by the fact that

the driver for the Arduino's USB-serial interface does not handle `XON`/`XOFF`

flow-control correctly.



The best way to send data to the plotter is to use a dedicated program that is

aware of the plotter's escape sequences, and can set up a workable flow-control

configuration itself. I wrote [hpplot](https://github.com/rhalkyard/hpplot) for

this purpose.



### Building `plotadapter`



1. Open `plotadapter.ino` in any recent version of the Arduino IDE



2. Using the Library Manager (Tools ➡ Manage Libraries...), install the

   "FreeRTOS" library



3. Open `config.h` and edit the pin definitions, plotter address, and other

   parameters to suit your setup.



## Notes



### Plotter Models



I wrote this project for my HP 7470A, but there is no reason this shouldn't work

for other similar plotters in the series.



### Software Architecture



`plotadapter` is implemented as a pair of tasks running on FreeRTOS -

`serial_task` reads bytes from the serial port, handles escape sequences, and

inserts received data into a ring buffer. `gpib_task` reads data from the ring

buffer, and transmits it via a very crude bit-banged GPIB implementation.



`gpib_task` also monitors the data stream for HPGL commands that are known to

generate output (any command beginning with `O` - `OI`, `OE`, `OD`, etc.). When

an output command has been sent, it sends a GPIB `TALK` command to the plotter

and waits for a response, which it sends to the serial port.



If a GPIB operation times out (the timeout is configurable as `GPIB_TIMEOUT` in

`config.h.`, default is 30 seconds), the bus is reset by asserting `IFC`, the

command buffer is purged, and `gpib_thread` is restarted. This also occurrs if

the "Abort Output" escape sequence `ESC . K` is received.



In hindsight, FreeRTOS is probably overkill for a simple project such as this,

and it could be rewritten using [protothreads](http://dunkels.com/adam/pt/) or

even just as a plain event loop. However, implementing GPIB and serial IO as two

independent tasks made it easier to reason about their interactions, and allowed

the GPIB implementation to be a simple piece of procedural code, rather than a

convoluted state machine. I regret nothing.



### Features



The following RS232 interface features are supported:



* DTR flow control (when using an external serial adapter)



* Echo mode setting with `ESC . @` (echo off, immediate echo, echo when character

  processed)



* Reporting available buffer space with `ESC . B`



* Reporting total buffer size with `ESC . L`



* Software low control (either `ENQ`/`ACK` or `XON`/`XOFF`) using Handshake

  Modes 1 (`ESC . H`) or 2 (`ESC . I`) - see section below on limitations



* Output-initiator and output-terminator characters



The following features are NOT supported due to architectural limitations:



* Reporting plotter status with `ESC . O` (querying plotter would interrupt HPGL

  command stream - returns status byte of `0` instead)



* Plotter enable/disable with `ESC . (` and `ESC . )`



The following features could be supported but I'm too lazy right now



* Inter-character and turnaround delay



* Echo-terminator and output-trigger characters



* Reporting RS232 error status with `ESC . E` (currently always returns `0`)



* Emulation of Circle and Arc HPGL commands that are present on RS232-equipped

  7470As, but not on GPIB models.



### Flow-Control Limitations



The driver for the onboard USB-Serial converter appears to ignore `XON`/`XOFF`

flow-control. Since we cannot use DTR flow control either, this needs to be

worked around by either:



* Attaching an external USB-serial interface (with funcitoning flow control) to

  the Arduino's serial pins



* Configuring/rewriting the application to use an alternate form of flow control

  (either `ENQ`/`ACK` or by checking available buffer space with `ESC . B`

  before sending)



### GPIB Bus limitations



This is *not* a fully-compliant GPIB implementation by any means - it works OK

if the plotter is the only device on the bus, but it will probably not be able

to drive a longer bus, and may not coexist well with other devices anyway.