Editing DECmate III

{{Infobox computer
| name         = DECmate III
| image        = [[File:DECmate III (Centre for Computing History).jpg|260px|frameless]]
| caption      = DECmate III system unit
| manufacturer = Digital Equipment Corporation
| type         = Word-processing and PDP-8 compatible personal computer
| release date = 1984<ref name="faq" />
| discontinued = 1990<ref name="faq" />
| cpu          = 6120 (PDP-8 instruction set)<ref name="faq" />
| memory       = 32K 12-bit words program memory plus a second bank for control panel functions<ref name="faq" />
| storage      = Integral RX50 dual 5.25-inch diskette drive<ref name="faq" /><ref name="mp39" />
| display      = VR201 monochrome monitor; VR241 colour with the graphics option<ref name="faq" />
| power        = H7845-AA (120 V) or H7845-AB (240 V), 78 W<ref name="mp39" />
| predecessor  = [[DECmate II]]
| model        = PC238 (print set); PC23P (sales model)
}}
The '''DECmate III''' is a compact word-processing computer made by [[:Category:Digital Equipment Corporation|Digital Equipment Corporation]] (DEC) from 1984 to 1990. It uses the same 6120 PDP-8 technology as the [[DECmate II]] in a smaller system unit with fewer expansion options.<ref name="faq" /> The unit holds a system board, an RX50 dual diskette drive and a power supply, and drives a VR201 monitor and keyboard.<ref name="faq" /><ref name="mp7" />

== History ==
Douglas Jones's PDP-8 FAQ gives 1984 for the introduction and 1990 for the withdrawal, and the sales model number as PC23P-Ax/LH, where x gave the monitor colour: A for black and white, J for green, S for amber.<ref name="faq" /> The FAQ explains the smaller design by DEC's finding that PDP-8 based products were rarely expanded to their full potential.<ref name="faq" /> DEC's print set for the machine, MP-01968-01, is titled PC238 DECmate III and dates its unit drawings from May 1984.<ref name="mp13" /> A later DECmate III+ (PC24P), from 1985, added a hard disk controller.<ref name="faqplus" />

== Hardware ==
[[File:DECmate III wiring interconnection drawing.png|thumb|right|340px|DEC's wiring interconnection drawing for the DECmate III unit: power supply (top), RX50 drive (lower left) and system board (right)]]
The unit is DEC's BA25-H small system box. Its parts list fits an RX50-AA dual 5.25-inch single-sided drive and an H7845-AA (120 V) or H7845-AB (240 V) supply, both described as 78 W small-system supplies; 85 W H7845-BA and H7845-BB supplies are listed as alternatives.<ref name="mp39" /> DEC's wiring drawing shows the supply cabled to both the system board (54-16489-01) and the drive.<ref name="mp7" />

{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+ '''DECmate III assemblies (print set MP-01968-01)'''
! Assembly !! Part
|-
| System board || 54-16489
|-
| Adaptor board || 54-16506
|-
| Diskette drive || RX50-AA
|-
| Power supply, 120 V || H7845-AA, 78 W
|-
| Power supply, 240 V || H7845-AB, 78 W
|-
| Enclosure || BA25-H
|}
<ref name="mp1" /><ref name="mp39" />

The system board takes its power through a 10-pin connector, J2: pin 8 carries +12 V, pin 7 +5 V, pin 5 −12 V, and pins 1 to 4 are ground.<ref name="mp23" />

=== Options ===
The FAQ lists a revised Z80 coprocessor (PC23X-AB/LH), a graphics board (PC23X-CA/LH) that let a VR241 colour monitor replace the monochrome one, and a modem (PC23X-DA/LH). Dual monitors were not supported, and the printer port is fixed at 4800 baud.<ref name="faq" />

== Known faults ==
* The VR201 monitor's electrolytic capacitors age. One restorer, working on a VR201 from a Rainbow, traced a shimmering picture to bulging or high-ESR capacitors and replaced six of them. The full list is on [[DECmate III Capacitor Replacement Guide]].<ref name="rob3" />
* RX50 drives can stop turning the spindle or lose their door pins. See [[DECmate III Troubleshooting Guide]].<ref name="rob3" />

== Documentation ==
The documents are described on [[DECmate III Documentation]].
* PC238 DECmate III print set, MP-01968-01: [[:File:DECmate III PC238 Print Set (MP01968-01).pdf]]
* VR201 monitor print set, MP-01410: [[:File:VR201 Monitor Print Set (MP01410).pdf]]

== Related pages ==
* [[DECmate III General Maintenance]]
* [[DECmate III Troubleshooting Guide]]
* [[DECmate III Capacitor Replacement Guide]]
* [[DECmate III Documentation]]
* [[DECmate II]]
* [[DECmate I]]

== References ==
<references>
<ref name="faq">Douglas W. Jones, "PDP-8 Summary of Models and Options", alt.sys.pdp8 FAQ, posted 8 June 2001, section "What is a DECmate III?", [http://www.faqs.org/faqs/dec-faq/pdp8-models/ faqs.org]. Dates, model numbers, configuration and options.</ref>
<ref name="faqplus">Douglas W. Jones, "PDP-8 Summary of Models and Options", section "What is a DECmate III+?", [http://www.faqs.org/faqs/dec-faq/pdp8-models/ faqs.org].</ref>
<ref name="mp1">Digital Equipment Corporation, PC238 DECmate III print set, MP-01968-01, drawing directory (PDF p. 1) ([[:File:DECmate III PC238 Print Set (MP01968-01).pdf]]).</ref>
<ref name="mp7">DEC, print set MP-01968-01, DECmate III unit assembly, wiring interconnection (PDF p. 7).</ref>
<ref name="mp13">DEC, print set MP-01968-01, unit assembly title sheet for the 120 V and 240 V DECmate III (PDF p. 13).</ref>
<ref name="mp23">DEC, print set MP-01968-01, system board schematic, power connector J2 (PDF p. 23).</ref>
<ref name="mp39">DEC, print set MP-01968-01, BA25-H small system box parts list (PDF p. 39). RX50-AA; H7845-AA 120 V and H7845-AB 240 V, 78 W; H7845-BA and -BB, 85 W.</ref>
<ref name="rob3">Rob Jarratt, "Rainbow 100A Restoration", ''Rob's Old Computers'', 29 July 2025, [https://robs-old-computers.com/2025/07/29/rainbow-100a-restoration/ robs-old-computers.com]. VR201 capacitors and cataracts; RX50 spindle and door, on a Rainbow 100A.</ref>
</references>

[[Category:Digital Equipment Corporation]]