add gpib plotadapter to accessories section

README.md

@@ -7,7 +7,7 @@ A curated list of code and resources for computer-controlled drawing machines an
 - [Hardware](#hardware)
   - [Plotters](#plotters)
   - [Motor Controllers](#motor-controllers)
-  - [Accessories](#accessories)
+  - [Accessories and Adapters](#accessories-and-adapters)
   - [Pens](#pens)
 - [Software](#software)
   - [HPGL](#hpgl)
@@ -47,10 +47,11 @@ A curated list of code and resources for computer-controlled drawing machines an
 - [Raspberry Pi CNC Hat](https://wiki.protoneer.co.nz/Raspberry_Pi_CNC) - Raspberry Pi add-on board w/ stepper controllers and a microcontroller running grbl. Interfaces with the Pi's serial pins.
 - [EBB Driver Board](https://shop.evilmadscientist.com/productsmenu/188) - USB-based dual stepper motor controller board, originally designed for the EggBot.
 
-### Accessories
+### Accessories and Adapters
 
 - [WiFi232](http://biosrhythm.com/?page_id=1453) - Wifi to RS-232 serial via a DB25 plug. Control your serial plotter wirelessly.
 - [Plotter Cable Pinout](http://sites.music.columbia.edu/cmc/chiplotle/plotter_cable.pdf) - Schematic for a plotter cable that will work for most HP and Roland plotters. Search eBay or Amazon for `DB9 to DB25 Serial Null Modem Cable` or similar to find them for sale.
+- [PlotAdapter](https://github.com/rhalkyard/plotadapter) - "Serial-GPIB converter for HP Plotters", uses an Arduino microcontroller to convert serial HPGL to the GPIB/HP-IB expected by some older HP plotters.
 
 ### Pens