New systems marked not working

------------------------------
Plexus P/20 [Dirk Best, misterblack1]

See https://github.com/misterblack1/plexus-p20

AI disclosure: gpt-5.6-sol was used as a research and debugging tool.

Author: startaq

src/mame/layout/p20.lay

@@ -0,0 +1,58 @@
+<?xml version="1.0"?>
+<!--
+license:CC0-1.0
+copyright-holders:Dirk Best
+-->
+<mamelayout version="2">
+	<element name="panel">
+		<rect>
+			<color red="0.08" green="0.08" blue="0.08" />
+		</rect>
+	</element>
+
+	<element name="title">
+		<text string="P/20">
+			<color red="0.9" green="0.9" blue="0.9" />
+		</text>
+	</element>
+
+	<element name="label0"><text string="0"><color red="0.9" green="0.9" blue="0.9" /></text></element>
+	<element name="label1"><text string="1"><color red="0.9" green="0.9" blue="0.9" /></text></element>
+	<element name="label2"><text string="2"><color red="0.9" green="0.9" blue="0.9" /></text></element>
+	<element name="label3"><text string="3"><color red="0.9" green="0.9" blue="0.9" /></text></element>
+	<element name="label4"><text string="4"><color red="0.9" green="0.9" blue="0.9" /></text></element>
+	<element name="label5"><text string="5"><color red="0.9" green="0.9" blue="0.9" /></text></element>
+	<element name="label6"><text string="6"><color red="0.9" green="0.9" blue="0.9" /></text></element>
+	<element name="label7"><text string="7"><color red="0.9" green="0.9" blue="0.9" /></text></element>
+
+	<element name="yellow_led" defstate="0">
+		<rect state="0"><color red="0.18" green="0.14" blue="0.01" /></rect>
+		<rect state="1"><color red="1.0" green="0.75" blue="0.05" /></rect>
+	</element>
+	<element name="green_led" defstate="0">
+		<rect state="0"><color red="0.01" green="0.16" blue="0.03" /></rect>
+		<rect state="1"><color red="0.05" green="0.9" blue="0.15" /></rect>
+	</element>
+
+	<view name="Terminal and LEDs">
+		<element ref="panel">
+			<bounds x="0" y="0" width="725" height="500" />
+		</element>
+		<screen index="0">
+			<bounds x="10" y="10" width="640" height="480" />
+		</screen>
+
+		<element ref="title">
+			<bounds x="667.5" y="9" width="50" height="16" />
+		</element>
+
+		<repeat count="7">
+			<param name="index" start="0" increment="1" />
+			<param name="y" start="36" increment="23.5" />
+			<element ref="label~index~"><bounds x="665" y="~y~" width="10" height="12" /></element>
+			<element name="led~index~" ref="yellow_led"><bounds x="682.5" y="~y~" width="22.5" height="12" /></element>
+		</repeat>
+		<element ref="label7"><bounds x="665" y="200.5" width="10" height="12" /></element>
+		<element name="led7" ref="green_led"><bounds x="682.5" y="200.5" width="22.5" height="12" /></element>
+	</view>
+</mamelayout>

src/mame/mame.lst

@@ -39792,6 +39792,9 @@ sslam
 sslama
 sslamb
 
+@source:plexus/p20.cpp
+p20
+
 @source:poly88/poly88.cpp
 poly88
 poly8813