Working documentation

This page is generated from the project's working notes. It records a real retrofit in progress, corrections included. Anything marked 🔴 or ⚠️ is unresolved or carries a caveat - read those before acting.

Mesa hardware - cards, cabling and how they chain

Answered 2 September 2026 by Peter Wallace («email redacted»), Mesa Electronics, in reply to Grant. Recorded here because the chaining answer decides both the parts list and the firmware.

The question put to Mesa

Grant listed what was already ordered, said he wanted to add a 7I77 for analog servo control, noted he already had the CAT5E-1-TB6 for the 7I76EU to 7I84U link, and asked two things: which cable connects the 7I77 to the 7I76EU, and whether all three cards can be connected together.

The answer

Question	Mesa's answer
Cable, 7I76EU to 7I77	DB25M-IDC26, a parallel cable. https://store.mesanet.com/index.php?route=product/product&product_id=315&search=db25m
Can all three chain?	Yes, using firmware 7i76eu_7i76x1_7i77x1d.bin on the 7I76EU. That firmware configures the 7I76EU's P1 expansion connector for compatibility with the 7I77 / 7I77U

Resulting topology

rp5800 (or its replacement)
  |  Ethernet, dedicated NIC
  v
7I76EU  -- P1 ------ DB25M-IDC26 -----> 7I77    analog servo command + encoder feedback
   |
   +---- SSerial --- CAT5E-1-TB6 -----> 7I84U   isolated remote field I/O

All three mount externally on DIN rail. Nothing goes inside the PC.

Parts list

Part	Qty	Unit USD	Status
7I76EU step/dir plus I/O, universal output	1	219.00	Ordered
7I84U isolated remote field I/O, sourcing/sinking outputs	1	79.00	Ordered
CAT5E-1-TB6 SSerial cable, 1 m	1	8.00	Ordered
DIN rail adapter kit (2 pcs)	2	8.00	Ordered
7I77U analog servo interface plus I/O daughtercard, universal output	1	229.00	✅ Ordered 2 Sep 2026
DB25M-IDC26 cable, 18"	1	10.00	✅ Ordered 2 Sep 2026
DIN rail adapter kit (3 pcs)	1	12.00	✅ Ordered 2 Sep 2026

Order record

Order ID	21237, store.mesanet.com
Date	2 September 2026
Contents	7I77U, DB25M-IDC26 18", DIN rail adapter kit (3 pcs)
Sub-total	USD 251.00
Shipping	USD 100.40, Priority Mail Express International
Total	USD 351.40
Status	Pending

Note the card ordered is the 7I77U (universal output version), not the plain 7I77. Same function; the U variant's outputs are configurable sourcing or sinking, which matters for the servo enables - see the polarity note below.

The Pn000.1 bet is now paid for, and it is a safe one either way. The card was bought before the drive parameter was read, so if Pn000.1 comes back as 1 (position, pulse train) the analog section goes unused. The card is still not wasted: the 7I77U brings 32 inputs and 16 outputs with it, which is exactly what closes the 9-output shortage. Under either control mode this was the board to buy. See servo-control-mode.md.

🔴 The 7I77U's universal outputs change the enable decision. The SGDH /S-ON input sinks (+24V-IN on CN1-47 feeds the opto commons, /S-ON pulls to 0 V). With a 7I77U in the chain the three servo enables can sit on its outputs configured sinking, rather than on the 7I84U or through interposing relays. That is the tidiest of the three options: the enable then lives on the same card as the analog command and the encoder feedback for each axis. See servo-drive-interface.md.

I/O totals with the 7I77 fitted

	Inputs	Outputs
7I76EU	32	16
7I84U	32	16
7I77	32	16
Total	96	48
TuChop needs (MW310.EQU, physical points)	41	41
	55 spare	7 spare

The machine requirement is 41 physical inputs and 41 physical outputs, not the 46 / 42 quoted in earlier notes. Those were symbol counts: IF.10 carries six symbols on one point, and OF.6 and OF.7 carry two each. Corrected 2 Sep 2026.

Grant's decision of 2 Sep to keep the drills and saw blades means all 41 outputs stay in use, so a third card is required regardless of which one it turns out to be.

~~Open question~~ - closed, no follow-up needed

An earlier note here raised whether the 7I84U's SSerial link survives the 7i76eu_7i76x1_7i77x1d.bin firmware. Dropped 2 Sep (Grant): Peter answered exactly the question asked - "Will I be able to connect all three cards together?" / "Yes, using that firmware." The 7I84U hangs off the 7I76EU's own SSerial port, which is part of that card's interface rather than a separate FPGA function the firmware name would list. No follow-up.

Related

• servo-control-mode.md - why Pn000.1 decides whether the 7I77 is needed at all
• sgdh-drive-reference.md - the CN1 pins the 7I77 would drive and read