add M27256 to docs

TommyPROM/TommyPROM.ino

@@ -41,10 +41,10 @@ PromDevice28C  prom(32 * 1024L, 64, 10, true);
 #elif defined(PROM_IS_27)
 // Define a device for a 2764 EPROM with the following parameters:
 //   8K byte device capacity
-//   PGM pin pulses active LOW
+//   Program using dedicated WR pin
 //   1000us (1ms) write pulse
-//   15 write attempts
-//   4x overwrite pulse
+//   Max 15 write attempts
+//   4x overwrite pulse (4 * writePulseLength * numberOfPulsesWritten)
 //   (true) verify data byte after writing
 //PromDevice27  prom(8 * 1024L, E27C_PGM_WE, 1000L, 15, 4);  // 2764 with SEEQ intelligent programming
 //PromDevice27  prom(32 * 1024L, E27C_PGM_WE, 1000L, 25, 3); // 27C256 with SEEQ intelligent programming

docs/_docs/design-250-prom-specific-code.md

@@ -37,6 +37,7 @@ different chip technologies.
 |AT29C010  |Atmel        |Flash  |28C    |Only with 128 byte or less sector size|
 |SST39SF040|Microchip    |Flash  |SST39SF|All SST39SF0x0 supported|
 |SST28SF040|SST          |Flash  |SST28SF|All SST28SF0x0 supported|
+|M27256    |ST Micro     |EPROM  |27     |VCC=6V, VPP=12.5V to pgm|
 |M27C256   |ST Micro     |EPROM  |27     |VCC=6.5V, VPP=12.75V to pgm|
 |W27C257   |Winbond      |EEPROM |27     |Continuous 12V or 14V for program/erase|
 |SST27SF020|SST          |Flash  |27     |12V continuous for pgm/erase|
@@ -213,6 +214,10 @@ while the programming voltages are present.
 
 This chip can only be erased with UV light, so the erase command is not supported.
 
+### M27256
+
+This is an older version of the M27C256.  Pin connections are the same, but for programming Vcc=6V and Vpp=12.5V.  The programming pulse width is 1ms instead of 100us, and the programming algorithm uses an overwrite pulse equal to 3ms * the number of program pulses written.
+
 ### W27C257 and W27C512
 
 The Winbond W27C257 and W27E257 appear to be identical 32Kx8 EEPROMs.  The 27C version

schematics/Adapters/card-edge/card-edge-32.kicad_pcb

@@ -17,8 +17,8 @@
     (33 F.Adhes user hide)
     (34 B.Paste user hide)
     (35 F.Paste user hide)
-    (36 B.SilkS user)
-    (37 F.SilkS user)
+    (36 B.SilkS user hide)
+    (37 F.SilkS user hide)
     (38 B.Mask user hide)
     (39 F.Mask user hide)
     (40 Dwgs.User user hide)
@@ -86,7 +86,7 @@
       (mirror false)
       (drillshape 0)
       (scaleselection 1)
-      (outputdirectory "./edge-card-32"))
+      (outputdirectory "./edge-card-32v2"))
   )
 
   (net 0 "")