\n\t\t\t\tVT220 rear panel\n\t\t\t\t<\/dd><\/dl>
\n\t\t<\/div>\n\nIt wasn’t easy to find VT220 documentation relating to the current loop interface. The Technical Manual (DEC P\/N EK-VT220-TM-001) doesn’t seem to show the pinout diagram. I was able to find the connector pinout in a hard copy of the VT220 Owner’s Manual (DEC P\/N EK-VT220-UG-003).<\/p>\n
Pin Signal\r\n\r\n 1 -12V\r\n\r\n 2 Transmit -\r\n\r\n 3 Receive -\r\n\r\n 5 Transmit +\r\n\r\n 7 Receive +\r\n\r\n 8 Ground<\/pre>\nLooking into the female 8-pin connector housing on the back panel of the VT220, Pin 1 is on the left and the two protruding plastic bumps (on the outside of the Mate-N-Lok female shell) are on the upper surface. Pin 8 is on the right. I confirmed this with a voltmeter by checking the -12V output pin.<\/p>\n
I then put a voltmeter across the TX pair (pins 2 & 5) and confirmed the voltage was 0.0V. I repeated this on the RX pair (pins 3 & 7) and got the same result. This tells me that the VT220 is a “passive” device. This means that although it can sense the current flowing in the RX loop, and it can open\/close the TX loop, it doesn’t actually provide a current source on either loop. So it requires that the device at the other end of the current loops (typically a PDP-8 or PDP-11\/10 computer) will be the “active” device on the loop. Each current loop requires one (and only one) active device on the loop.<\/p>\n
The “Transmit” and “Receive” designations are relative to the VT220. So “Transmit” means data being transmitted by the VT220 terminal to the computer.<\/p>\n
DEC’s conventions for current loop connectors<\/h3>\n
From the information above I have been able to figure out the following about DEC’s conventions for current loop wiring:<\/p>\n
\n- Each piece of DEC equipment (whether a computer, printer or terminal) would have an 8-pin female Mate-N-Lok connector on it<\/li>\n
- A “null modem” cable, with male 8-pin Mate-N-Lok connectors on each end of it, would be used to connect any 2 pieces of equipment together<\/li>\n
- You could save a few connectors (as the ASR33 did) by not putting a Mate-N-Lok connector on the device itself. Instead, these devices would run a longer cable (permanently attached to the device) direct to an 8-pin male Mate-N-Lok connector at the other end of the cable, which would mate directly with the 8-pin female Mate-N-Lok connector on the DEC equipment<\/li>\n<\/ol>\n
The null-modem cable<\/h3>\n
To connect two devices that have 8-pin female Mate-N-Lok connectors we need a 4-conductor cable that “swaps” the TR and RX pairs, as shown below:<\/p>\n
\n\n\n| Active end (computer)<\/th>\n | Passive end (terminal)<\/th>\n<\/tr>\n |
\n| 5 (Transmit +)<\/td>\n | 7 (Receive +)<\/td>\n<\/tr>\n |
\n| 2 (Transmit -)<\/td>\n | 3 (Receive -)<\/td>\n<\/tr>\n |
\n| 7 (Receive +)<\/td>\n | 5 (Transmit +)<\/td>\n<\/tr>\n |
\n| 3 (Receive -)<\/td>\n | 2 (Transmit -)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n Cables that are intended to connect a computer to an ASR33 need an additional two wires (for the reader enable current loop). However, it doesn’t make sense to include these in a null-modem cable, as they are unidirectional (the PDP-11 is always the transmitter, and the ASR33 is always the receiver). So don’t connect the reader-enable wires in a null-modem cable.<\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" This post takes a brief look at the current loop connectors used by DEC. The main focus of this post is the cabling needed to connect a VT220 terminal to a computer (such as a PDP-11\/10) using the 20mA current loop interface. DEC’s standard connector DEC used an 8-pin Mate-N-Lok connector for all current-loop cabling ..<\/p>\n <\/div>\n Read more<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":true,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[10],"tags":[],"class_list":["post-252","post","type-post","status-publish","format-standard","hentry","category-current-loop"],"jetpack_publicize_connections":[],"jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p7xTsk-44","jetpack-related-posts":[{"id":794,"url":"https:\/\/avitech.com.au\/?p=794","url_meta":{"origin":252,"position":0},"title":"RS232 connectors","author":"malcolm","date":"20-July-2016","format":false,"excerpt":"I've just put up a webpage with some reference information about various RS232 connectors, including the DEC423 MMJ connectors. If interested, please take a look here -> http:\/\/avitech.com.au\/?page_id=766.","rel":"","context":"In "DEC"","block_context":{"text":"DEC","link":"https:\/\/avitech.com.au\/?cat=2"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":137,"url":"https:\/\/avitech.com.au\/?p=137","url_meta":{"origin":252,"position":1},"title":"DECserver 200","author":"malcolm","date":"7-May-2016","format":false,"excerpt":"What is it? The DECserver 200 is a \"terminal server\". It enables up to 8 terminals to login to various DEC computer systems over an Ethernet link. Back in the day a DECserver it was used to connect terminals and printers to DEC systems.\u00a0 Typically these were VAX machines, though\u2026","rel":"","context":"Similar post","block_context":{"text":"Similar post","link":""},"img":{"alt_text":"ds200-front-view","src":"https:\/\/i0.wp.com\/mal-home.dyndns.org\/wordpress\/wp-content\/uploads\/2016\/05\/ds200-front-view-1024x576.jpg?resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/mal-home.dyndns.org\/wordpress\/wp-content\/uploads\/2016\/05\/ds200-front-view-1024x576.jpg?resize=350%2C200 1x, https:\/\/i0.wp.com\/mal-home.dyndns.org\/wordpress\/wp-content\/uploads\/2016\/05\/ds200-front-view-1024x576.jpg?resize=525%2C300 1.5x, https:\/\/i0.wp.com\/mal-home.dyndns.org\/wordpress\/wp-content\/uploads\/2016\/05\/ds200-front-view-1024x576.jpg?resize=700%2C400 2x"},"classes":[]},{"id":605,"url":"https:\/\/avitech.com.au\/?p=605","url_meta":{"origin":252,"position":2},"title":"Imaging DEC VAX\/VMS CDs","author":"malcolm","date":"10-June-2016","format":false,"excerpt":"I have some original VMS CDROMs. I was looking for a way to image these (for use with SIMH) and also a way to write ISO images back to blank CDs, to use on real hardware. This should be a fairly simple process using a modern Linux computer and the\u2026","rel":"","context":"In "VMS"","block_context":{"text":"VMS","link":"https:\/\/avitech.com.au\/?cat=5"},"img":{"alt_text":"vaxvms061cd","src":"https:\/\/i0.wp.com\/avitech.com.au\/wp-content\/uploads\/2016\/06\/vaxvms061cd-300x300.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":3749,"url":"https:\/\/avitech.com.au\/?p=3749","url_meta":{"origin":252,"position":3},"title":"Freshly scanned manual for MDB MLSI-CR11 Card Reader Controller","author":"malcolm","date":"11-December-2019","format":false,"excerpt":"I came across a hardcopy manual for MDB's MLSI-CR11, which is a QBUS Card Reader Controller. It appears to be similar in function to the DEC CR11 UNIBUS controllers (M8290\/M8291) except that it has a 50-pin I\/O header instead of 40 pins. Freshly scanned manual can be found here ->\u2026","rel":"","context":"In "PDP-11"","block_context":{"text":"PDP-11","link":"https:\/\/avitech.com.au\/?cat=11"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/avitech.com.au\/wp-content\/uploads\/2019\/12\/mdb-mlsi-cr11-front-cover-232x300.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":3632,"url":"https:\/\/avitech.com.au\/?p=3632","url_meta":{"origin":252,"position":4},"title":"Webster Spectrum PDP-11\/73","author":"malcolm","date":"6-October-2019","format":false,"excerpt":"This is a PDP-11\/73 in a Webster Spectrum enclosure. The 11\/73 board (M8192) appears to be the only genuine DEC board in the backplane. The Spectrum is mounted in a half-height rack that also houses a Cipher tape drive. The Cipher tape drive is a Model F880640-90-1025U, with Option E\u2026","rel":"","context":"In "PDP-11"","block_context":{"text":"PDP-11","link":"https:\/\/avitech.com.au\/?cat=11"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/avitech.com.au\/wp-content\/uploads\/2019\/10\/IMG_20191006_194737.jpg?resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/avitech.com.au\/wp-content\/uploads\/2019\/10\/IMG_20191006_194737.jpg?resize=350%2C200 1x, https:\/\/i0.wp.com\/avitech.com.au\/wp-content\/uploads\/2019\/10\/IMG_20191006_194737.jpg?resize=525%2C300 1.5x, https:\/\/i0.wp.com\/avitech.com.au\/wp-content\/uploads\/2019\/10\/IMG_20191006_194737.jpg?resize=700%2C400 2x, https:\/\/i0.wp.com\/avitech.com.au\/wp-content\/uploads\/2019\/10\/IMG_20191006_194737.jpg?resize=1050%2C600 3x, https:\/\/i0.wp.com\/avitech.com.au\/wp-content\/uploads\/2019\/10\/IMG_20191006_194737.jpg?resize=1400%2C800 4x"},"classes":[]},{"id":220,"url":"https:\/\/avitech.com.au\/?p=220","url_meta":{"origin":252,"position":5},"title":"PIC USB bootloader","author":"malcolm","date":"13-May-2016","format":false,"excerpt":"I've got a couple of projects on the go that use the PIC18F2550 (or PIC18F2553) microcontrollers. Normally I program PICs in-circuit with a PICkit 3 programmer while debugging my code. While this works great, I also need a way for people to easily field-upgrade my PIC firmware, without the need\u2026","rel":"","context":"In "PIC"","block_context":{"text":"PIC","link":"https:\/\/avitech.com.au\/?cat=4"},"img":{"alt_text":"hid-bootloader-application-screen-shot","src":"https:\/\/i0.wp.com\/avitech.com.au\/wp-content\/uploads\/2016\/05\/hid-bootloader-application-screen-shot.jpg?resize=350%2C200","width":350,"height":200},"classes":[]}],"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/avitech.com.au\/index.php?rest_route=\/wp\/v2\/posts\/252","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/avitech.com.au\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/avitech.com.au\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/avitech.com.au\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/avitech.com.au\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=252"}],"version-history":[{"count":9,"href":"https:\/\/avitech.com.au\/index.php?rest_route=\/wp\/v2\/posts\/252\/revisions"}],"predecessor-version":[{"id":1405,"href":"https:\/\/avitech.com.au\/index.php?rest_route=\/wp\/v2\/posts\/252\/revisions\/1405"}],"wp:attachment":[{"href":"https:\/\/avitech.com.au\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=252"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/avitech.com.au\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=252"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/avitech.com.au\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=252"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}} |
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