How to prepare the UART cable’s wires (Compute Blade)¶
You have the parts for an Acorn on a Compute Blade and are making the second of its two cables, from the Acorn’s P2 socket (the serial port). On this page: find wire 1, check it with a meter, cut back the wires that are not used, solder the resistor into the J2 wire, crimp the rest.
This procedure is waiting for its bench run: issue #219.
Photos: Uptime Lab (Compute Blade), RHS Research (LiteFury underside; the Acorn is the same PCB).
What you need¶
Parts
the other half of the Molex Pico-EZmate cable, which was cut in half in How to prepare the JTAG cable’s wires (Compute Blade)
the 1×4 Dupont housing
3 Dupont crimp terminals, and a few spare
2 mm heat-shrink tube
the 470 Ω resistor
3 mm heat-shrink tube
the Acorn
Tools
a multimeter with a continuity buzzer, and a fine probe tip for it or a sewing pin to hold against a probe
a ruler marked in millimetres
side cutters, wire strippers and the crimping tool
a hot-air tool
a soldering iron
masking tape and a fine pen
a PH1 (Phillips size 1) screwdriver
Steps¶
1. Have the Acorn out of its slot. If it is fitted, the Compute Blade must be off first. Ask the site operator, then power off the Compute Blade: unplug its PoE cable, and a USB-C cable if one is plugged in.
Then take the card out. Take its screw out first: an M2x2.5 screw with an M2 nylon washer, which takes a PH1 driver (the Compute Blade’s assembly guide). Leave the M2x2.5 standoff in its threaded insert.
The Acorn is a bare circuit board. Before you pick it up, touch bare metal of the host, or wear an anti-static wrist strap; hold the card by its edges.
If the Molex cable is still whole, do the reach check first: step 2 of How to prepare the JTAG cable’s wires (Compute Blade). Then cut it in the middle with side cutters: each half is one cable.
2. Find wire 1 of the P2 cable and flag the wires, before cutting any wire back.
Press the plug of the other half into socket P2 on the underside of the Acorn. Hold the card underside up with the M.2 edge to your left: wire 1 is the leftmost. In the picture under this step, the card in the photograph has a cable already in P1 and nothing in P2. Your card has nothing in either socket until you press a plug in.
Put a numbered tape flag on each of the 6 wires, 1 at the left to 6, about 25 mm from the cut end. Keep each flag clear of the end that will be cut and stripped later. Write P2 on the flag of wire 1 as well: off the card, the two halves look alike.
3. Check which wire of the P2 cable is wire 1, with a meter, before cutting any wire back.
With the plug still in the socket (the Acorn out of any slot), set the meter to continuity. Press the probe tip, or a pin held to it, against the cut face of the wire flagged 1 (it is not stripped yet). Put the other probe on the plated half-round mounting pad at the end of the card, not on the round holes. It must beep.
Do the same with the wire flagged 6: it must stay silent.
If wire 6 beeps instead, stop: the numbering is reversed; take the flags off and number from the other end. Then do items 1 and 2 of this step again, with the flags renumbered. If neither wire beeps, strip about 3 mm from wires 1 and 6 and try again. If still neither beeps, stop: the ground point is not confirmed.
If both wires beep, first see that the two probes do not touch each other. See also that the cut faces of wires 1 and 6 do not touch. If both still beep, stop and cut nothing: the check cannot tell the wires apart. Leave the flags on and take the plug out. Wire 6 is the card’s 3.3 V by the LiteX wiki’s legend. It is expected to stay silent to ground on a card with no power. Set the meter to ohms. Write down what each of the two wires reads to the pad. Send both readings to the site operator.
Take the plug out again.
4. Cut wires 4, 5 and 6 off about 10 mm from the plug. Shrink a piece of the 2 mm tube over each cut end. They go in no cavity. Wire 6 is VCC, 3.3 V from the Acorn by the LiteX wiki’s legend: it must never reach the host. Leave wires 1, 2 and 3 at full length.
5. Cut wire 3 (J2) about 15 mm from its free end. Slide a piece of the 3 mm tube onto the wire, clear of the cut. Strip about 3 mm from each cut end. Trim the resistor’s leads to about 5 mm each.
Solder the 470 Ω resistor between the two cut ends. Slide the tube over the resistor and both joints and shrink it. Crimp the terminal only after this.
The resistor is there because J2 lands on GPIO14, which is also JTAG TMS. The 470 Ω in the wire is meant to let JTAG through if the FPGA drives J2.
6. Strip about 3 mm from wires 1, 2 and 3. Crimp a Dupont terminal on each.
Check¶
Wire 1 of the P2 cable is the one the meter check of step 3 found. Each wire that goes in the housing carries its numbered flag and has a terminal crimped on.
If it fails¶
The meter check of step 3 finds the numbering reversed, or finds both wires beeping: that step says what to do, and when to stop.