Here’s the user manual :
https://www.humminbird.com/sites/hum...390.1638919911
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The AS PC3 connect cable kit was used in these older units for updating software before SD Cards were prominent ...
HB shows out of stock but some online retail outlets may still have it ... don’t rule out big box stores like Walmart.com or Amazon when searching ...
https://humminbird.johnsonoutdoors.c...connection-kit
Check here for the latest software version for Matrix 27 :
https://www.humminbird.com/support/s...rix-27-v-3-840
Rickie